The Mishnah in Sukkah 2:9 gives a parable to the situation of rain falling on Sukkos:
מָשְׁלוּ מָשָׁל, לְמָה הַדָּבָר דּוֹמֶה, לְעֶבֶד שֶׁבָּא לִמְזוֹג כּוֹס לְרַבּוֹ, וְשָׁפַךְ לוֹ קִיתוֹן עַל פָּנָיו:
To what can this matter be compared? To a slave who came to pour a goblet for his master, and he [the master] poured a bowl-full [of water] in his face.
Rambam explains:
וירידת הגשמים בתחלת הסוכות רמז כי השם אינו מקבל מעשיהם ברצון:
The falling of rain at the beginning of sukkos is a hint that Hashem doesn't accept their deeds with pleasure.
The Rambam implies that the "simman klala" of rain on Sukkos is only at the beginning of sukkos. Presumably he means since only the first night it is obligatory to eat in the sukkah. Other days are not obligatory to eat, so if it rains it's not as big a deal.
My question is from the first Mishnah in Taanis:
מֵאֵימָתַי מַזְכִּירִין גְּבוּרוֹת גְּשָׁמִים. רַבִּי אֱלִיעֶזֶר אוֹמֵר, מִיּוֹם טוֹב הָרִאשׁוֹן שֶׁל חָג. רַבִּי יְהוֹשֻׁעַ אוֹמֵר, מִיּוֹם טוֹב הָאַחֲרוֹן שֶׁל חָג. אָמַר לוֹ רַבִּי יְהוֹשֻׁעַ. הוֹאִיל וְאֵין הַגְּשָׁמִים אֶלָּא סִימַן קְלָלָה בֶּחָג, לָמָּה מַזְכִּיר. אָמַר לוֹ רַבִּי אֱלִיעֶזֶר, אַף אֲנִי לֹא אָמַרְתִּי לִשְׁאוֹל, אֶלָּא לְהַזְכִּיר מַשִּׁיב הָרוּחַ וּמוֹרִיד הַגֶּשֶׁם בְּעוֹנָתוֹ. אָמַר לוֹ, אִם כֵּן, לְעוֹלָם יְהֵא מַזְכִּיר:
From when do we begin to mention the power of [bringing] rain [in the Shemoneh Esrai], Rabbi Eliezer says, from the first day of the festival [Sukkot]. Rabbi Yehoshua says, from the last day of the festival [Sukkot]. Rabbi Yehoshua said to him [Rabbi Eliezer], since rain on the festival is nothing but a sign of a curse why should he mention it? Rabbi Eliezer said to him: I also did not say to request [rain] but to make mention, “He causes the wind to blow and the rain to fall” in its due season. He replied to him: if so one should at all times make mention of it.
Rabbi Yehoshua intimates it's inappropriate to mention rain during sukkos, since it's a simman klala. All Rambam there says is:
מאימתי מזכירין גבורות גשמים רבי אליעזר אומר מיום טוב הראשון כו': כבר קדם לך בסוף הפרק השני מסוכה כי ירידת הגשמים בסוכות אינו סימן ברכה. והלכה כרבי יהושע:
The Mishnah at the end of the second chapter of Sukkah already explained to you that rain falling on Sukkos is not a sign of blessing. The halacha is like Rabbi Yehoshua.
How does the Rambam read this Mishnah? Why does Rabbi Yehoshua say it's inappropriate to mention rain on Sukkos, since it's a simman klala, if it's only a simman klala tlat the beginning? Why not mention it after the first day(s)?