As the title says, is it permissible to respond "Amein" and "B'rik Hu"to a Kaddish/Chatzi Kaddish prayer made in gibberish, with words like "Blah blah blah" thrown in for jest? Or should one stay silent in such cases?
Your question is not clear, but:
If it's an actual prayer, then added in "sounds" are acceptable. It's not unusual for a Chazzan to add in la-lala-la or similar - so as to stretch the words to the tune. Answering Amein is correct.
If it's a parody that simply sounds like a prayer then there's a problem answering Amein (called an orphaned Amein - one without a prayer preceding it). Depending on circumstances it may also be in bad taste. (Apparently, nothing is in bad taste on Purim, according to some.)
Shulchan Aruch (OC 215:4) writes:
כל המברך ברבה שאינה צריבה הרי זה נושא שם שמים לשוא והרי הוא כנשבע לשוא ואסור לענות אחריו אמן
Anyone who says a blessing that is not necessary is mentioning Hashem's name in vain and is like swearing falsely, and it is forbidden to answer Amen after it.
In addition, the Taz (OC 560:5) writes that it is a great sin to sing Kaddish at a meal, and all the more so to make a joke out of pesukim or holy words.