In Paper Mario: The Thousand-Year Door, Goombella attends a university called U Goom. While this is not a pun on any specific university in English, other languages make it a Goomba version of various real-life universities.

She graduated all these top universities in at least 7 different languages? No wonder she was allowed to cameo in Sticker Star!

i thought old paper mario stuff was Verboten since sticker star, very interesting

Yeah, that's definitely not the case.

Appearing in game is at least (outside of Smash trophies/spirits and stuff)

This counter-example immediately came to mind.

That’s Peach unless I’m missing something

Yeah, Peach from PM64, PM: TTYD, & SPM.

I meant the PM original characters like Goombella (and anything else that breaks the “mandates”), not anything from the games period. By that logic Mario couldn’t even be in his own game because he uses his TTYD design with a white outline and has a hammer. Lol