It depends, the nice guy might not necessarily be a simp, and the simp might not be nice.
A simp is someone who's doing maximum effort for little to no reward. Basically thirstin for unrequited feelings. But a girl wouldn't necessarily criticize a simp cause that's useful to her, especially if she can get a work around of her not having to express feelings. So this "nice" guy could be going out his way to give benefits to the girl, either physical or through socializing.
Nice is a pretty generic term. What is "nice"? That's basically just simply not being a jackass. Anybody can be nice, so that's not really a thorough comment. Uniqueness is to be celebrated, comments that show you are unique to her. Nice don't do that.