The truth is proportions are relevant no matter what style someone is drawing in. Stylized work still has to look "good" in it's own sense, and part of that comes from an understanding of anatomy and such, even if they're not following realistic human proportions. Knowing that, I think it's relatively obvious when its actually someones style and when its just a novice artist being defensive.
Though I think whether or not the artist cares about getting better and is asking for a critique is relevant too. It's rude to ask for critique and then use your style as a way to dismiss it. I would call that a problem of ego, and hopefully something said artist will learn not to do! At the same time, I've seen unsolisited critics tell young artists they shouldn't draw or develop a style until they've mastered life drawing, which is kind of insane to me. You can study the fine art stuff and come home to doodle big eyed bobble headed whatevers if you want.
I don't think it's the style that's a crutch. It has everything to do with what the artist wants and how much they care. If they're drawing in a style, that's not an excuse not to get better from practice.
And its worth keeping in mind art is one of those skills where you tend to never feel good enough but are always getting better. It can be emotionally exhausting and crippling, especially for artists that might be struggling with their own demons. As a rule of thumb, artists should do as they like. BUT they also shouldn't complain about how they're not getting better fast enough if their not putting in the work.
I'm sorry, this feels like it went long and rambling-y.