Please explain the following Gemmorah:
ואמר רבה א"ר יוחנן עתיד הקב"ה לעשות סוכה לצדיקים מעורו של לויתן
And Rabba says that Rabbi Yoḥanan says: In the future, the Holy One, Blessed be He, will prepare a sukka for the righteous from the skin of the Leviathan (whale or giant fish) (B"B 75a)
Why should we sit in Succah made of a giant fish skin?
This text is clearly allegorical and is not to be taken in the literal sense. Also, your translation of Leviathan as whale or giant fish is only a modern idea. It is not how traditional Judaism understands this term.
If you look in the commentary from Sefer Pri Tzaddik concerning this section of Gemmorah, it explains that these creatures and their associated skins are allegorical references to the different aspects of the Yetzer HaRa and how, in the final redemption, they will be utilized in their original, appropriate manner intended by G-d.
וזה ענין הסעודה שיעשה לצדיקים מלויתן דהיינו שיושאר המטעמים חמידו דאורייתא שע"ז התכלית נברא יצר של התאוה שיהי' חשק לד"ת. וכן א' בגמ' (ב"ב עה.) עתיד הקב"ה לעשות סוכה לצדיקים מעורו של לויתן כו' ומשור הבר לא מצינו שיושאר מה. והוא שמחשק התאוה יושאר בקדושה להיות חמידו דאורייתא וז"ש והשאר פורסו הקב"ה על חומות ירושלים וזיוו מבהיק כו'.
I hope this will answer your question: The answer is taken from a sefer called Hakhmei Lev. Its called "The Meaning and Significance of the Leviathan" written by Rabbi Ken Stollon. It can be found on page 175.
I'll include part of the last paragraph which addresses your question, but its also important to read the entire thing to understand how he came to the conclusion.
"The great sea creature, which, as I have argued, is the living embodiment of our big, unfathomable questions, will be cut up into small, digestible servings for us to ingest and digest. In other words, metaphorically, the big questions in life will be broken down and made comprehensible to us. We will literally get under the skin of these questions; when we sit in the sukkah made out of the skin of the Leviathan, we will come to know the Leviathan outside-in and inside-out; we will, in a sense, be fully immersed in the Leviathan. So, too, will we, in the End of Days, basking in the glow of Divine knowledge, be fully enlightened to all the mysteries of the universe, and merit a brand new understanding of all the large questions, the painful and perplexing questions, in our lives."
In ספר ימי דוד, the author (p.109) cites a posuk in Iyov, where it says:
canst thou fill his skin with barbed irons? or his head with fish spears?
On this posuk, the Shelah HaKadosh explains the significance of the Leviathan:
From all the above it is clear that the skin of Leviathan is a very great light indeed. I have found a statement in the commentary of Tziyoni on this portion, based on a Midrash that the skins used to make the כתנות עור for Adam and Eve were taken from the female of the original Leviathan, whom G–d reportedly slew, salting and preserving its "meat" for the righteous in the future (Baba Batra 74). This seems very plausible; G–d surely did not create that skin without assigning a purpose to it.
According to the Shelah HaKadosh, the garment that Adam HaRishon wore was made from the skin of the Leviathan:
If we accept that Midrash as being factual, we can much better understand the statement that in Rabbi Meir's version of the Torah the word Or in Kotnot Or was spelled with the letter א, light. It would have been difficult to imagine that the spelling in Rabbi Meir's ספר תורה was really so radically different. What Rabbi Meir meant by his statement was that the skin of the Leviathan which formed the clothing of Adam illuminated the earth all over. (emphasis mine)
Maybe what this is teaching us is, as the Shelah explains, that we are given access to all the blessings in the world.
In the sefer סיפורי חז"ל מתלמוד בבלי עם הערות וביאורים it is explained that this Gemara is to be taken literally. G-d will show all other people that he rewards those who held on to His Torah, no matter what. He will give those who hold unto his Torah protection, literal protection.
The author goes on to cite the Maharsha (ad.loc) who seems to explain that there are different degrees of rewards, with regard to the "skin of the Leviathan". Every person, according to his degree, gets a bit of the skin of Leviathan. Only those that fulfill mitzvos out of love for G-d and with no other motives, will get rewarded in the form of a "full Leviathan skin". - to be honest, I do not know how to interpret this. It seems that the Gemara in Bava Basra is speaking about those people the author speaks about, the people that deserve the fullest protection and reward for learning and observing Torah lo-lishmah.
See also sefer אור אברהם
The Maharsha there says that just like the meat is eaten specificly in Yerushalaim, which he interprets to be as food bought with money of maser shaeni which is a mitzvah, also the skin will be used for a mitzvah. See the explanation inside, I am summarizing.
Why for a succa and not, say, for writing sifrei torah? It probably has to do with the end of the Gemara there, that the skin will be used to cover the walls of Yerushalaim.
(BTW, according to this it is clear that the walls will be from Livyoson skin but there roof will be kosher schach.)
If you insist on taking this literally as being about material, tents (which some translate 'succah' as) have historically been made of animal skins, and you can also get fish leather, so Leviathan's skin might not necessarily be unsuitable.
It depends on exactly what kind of animal the Leviathan is understood to be - The Gemara uses 'salted fish' as a comparison for the preserved female, and the rabbis of that era might well have understood whales as giant fish, but honestly the descriptions often sound closer to a dragon - Which can be an umbrella term, so that doesn't exclude it from also being technically a fish, mammal or reptile (the appellation 'Rahab' is sometimes translated as 'crocodile'). Supposedly we will eat its flesh, but some believe the laws of kashrut will no longer apply, so that doesn't narrow it down much (note that actual whale skin is eaten amongst some people, such as the 'muktuk' of Greenland, but whale skin - unlike whale bones - specifically doesn't seem to be used for construction).
As for 'Why not all-loved cows', well quite simply, this is one very large and supposedly luminously splendid skin to divide amongst millions of people for sukkot, head-dresses, covers, necklaces (presumably like a leather thong?) and amulets, depending on the person. In comparison, it would take a ridiculous number of cows, plus the idea that this one material being good for all those things is probably meant to indicate its WTC-esque superiority. Cowhide wasn't luminous at my last check, and most people understood cowhide tents as the earthly norm. This is all supposed to take place during the 'Days of Moshiach' so many ideas about it indicate novel experiences and fantastical standards rather than mundane, everyday ones. Much like how the heavens are imagined to be paved with sapphire rather than with the limestone of Jerusalem painted blue.
If you are concerned about the smell (tanning cow hides also smells bad), well I imagine G-d is supposed to take care of that. If you would really prefer a cowhide sukkah, maybe ask G-d to make you one from the hide of the Behemoth (which I'm pretty sure is smaller than Leviathan) instead, whose meat we will also supposedly feast on? His skin doesn't seem to have a designated use yet (although like whale skin, cow skin can be made edible).
No word on whether we get to eat/utilise the Ziz either...
