Some chick my cousin was hanging out with once called some random waiter "senpai"

I'm not an expert but I'm 99% sure that's a misuse of the term "senpai."

Also, question for the artist formerly known as "Boiled Banana:" is your new name a reference to something?

Senpai is like someone above you. A mentor or someone you look up to.
I could be wrong

(Yes it was a misuse of "senpai"
No "Acid_Freak" is not a reference to anything, i made it up as a name of a track on music generator for ps1, one of the sounds had the word "acid" in it and it was one of the main sounds foe the track i was making, so i decided to name it acid freak, then around maybe a year later i decided to make a bandlab account and the first name that came to mind was "Acid_Freak".

I wonder if any Japanese people ever feel weird saying the word baka after everyone kept making it a meme

I actually hate that word now.

Dude it's like a whole other language is word you can't exactly hate it, i mean if you ever went to Japan right if a person who speaks Japanese said it you can't exactly just tell them to stop saying it because you don't like how it sounds, it's like if I asked my aunts to stop saying like "cantik" when they're describing something is pretty because that's how they say pretty in Indonesian