Monika wants to be in Smash, but not Fortnite

Super Smash Bros. was made in the 90s...
I think you may be referring to Super Smash Bros. Ultimate, which in that case you can't because the only characters that are allowed in the game would be Nintendo characters only!
But there's a much greater chance of being added to Fortnite, because there's no category-boundaries to the game...

however if 5 year olds see it out of fortnite (like for example the game she originally appeared in) they might say that they stole her from it. also the made a mii gunner for sans too remember?

Sans is technically Nintendo cause it’s on the switch

DDLC+ is also on the switch