Hmm you got a point there, the touching could be a big disadvantage of the illusion, but maybe, if you don't want her to make it an overpowered illusion ability, is a good way to balance it.
I'm just gonna say some random ideas, some might sound bit silly XD but I dunno, maybe part of them could help even a bit. But feel free to ignore me, I just sometimes like given random ideas.
Maybe how strong the illusion is depending on who is seeing it, like, there are some people that are easier to fool and even when they try to touch, their minds are weak so they can’t tell it’s not real unless exposed for really long periods. Then there could be other minds, stronger or people not so easily tricked that might see the illusion but upon touching, will fade. There could even be some that the normal illusion wouldn’t work so well, like other supernatural beings or slenders.
Now that you mention it, I actually forgot about Sulker dark sense of humor XD but if Joy has a similar humor sense, it might actually be interesting to see them interact (it would be interesting to see Sulker interact without trying to kill the other in general XD).
He also respects slenders or other beings with cruel mentalities or ways of controlling others, so I bet he’d find interesting Joy illusions and all that.
Sulker would probably just be amused by her dark jokes, in an extreme case, darkly chuckle to himself, dropping a sarcastic or same type of joke himself now and then.
He probably wouldn’t be a “fan” of Joy happier side, but as long as she isn’t as bubbly or annoying, he could stand her at times.
This could be a very interesting allies thing, I just love how different n opposite they look XD
I’m gonna go add Joy as Sulker Allies~