If you want to go to a potluck dinner, but don’t want to or can’t cook anything, ice cream can be a good thing to bring.
There are other things people who don’t want to or can’t cook sometimes bring, and they’re not rude exactly, but people who bring those things are often perceived as lazy or…
I often bring fruit! People seem to like that. Unless they’re just being polite. But it too has the virtue of being unexpected, and it’s a nice addition to a dessert.
I actually think fruit is a brilliant idea. After eating a bunch of (often heavy, filling) dishes at a potluck, fruit is really refreshing. Almost like a palate cleanser.