"They're not? Why not?"

Add: Agree. I'm truly baffled by the parent comment.

People say things about appearance all the time. And I can't think of many cases where a comment (positive) about how someone appears (if said in the proper tone of voice [1]) is not appropriate. And even in the case of a negative comment you'd have to take the relationship of the parties into account (agreeing that that's a much broader situation and safer to not say anything).

[1] Of course I can think of many other exceptions (person dying in the hospital and you say "you look great") but the parent comment doesn't appear to be suggesting fringe cases.

It's not necessarily a bad thing, but you should consider that it's often unwelcome. There are things people think of as compliments to me that aren't things I actually want to hear, particularly from people I don't know.