Hi, dolco. As you said, you got the meaning, so let me try to give you a general idea on the structure. In "to be all looks", "looks" is actually a noun that means appearance. "All" here acts as an adverb of sorts, like "completely". Loosely speaking, an explanation for the structure you mention is here: https://www.phrasemix.com/phrases/someone-is-all-about-something The difference, in this case, is that "about" is implicit. She's all (about) her looks/looking gorgeous. I hope this helps you. :) Regards.

@Robert Thank you so much! I really appreciate it

Your statement, She's kinda girl who is all looks. is not grammatically correct, although it might sound like that if spoken It should be She's THE kind OF girl who is all looks.

@Rosco Thanks for the comment :D

What it means is she is all looks and no brains or anything else. Or all she cares about is her appearance.