Why not "wiecej popularny"? What is the difference between bardziej i więcej? Thank you!
"więcej" is about the amount, "bardziej" is about the... degree? intensity?
So frankly, unless I'm forgetting something right now, I think we can assume that "więcej" goes with nouns or verbs and "bardziej" goes with adjectives and some adverbs.
"Kto jest popularniejszy?" is another perfectly acceptable version
It is, and we accept it.
Czy można powiedzieć 'słynny' zamiast 'lubiany'?
That's a totally different thing. My friend may be a really popular person in school, but he's not 'famous' in any way...
Is 'kto' masculine?
Yes. Even if you asked "Who did it?!" in a female boarding school, it's still grammatically masculine.
Is there a way to say "Who did it?" such that "popularna" would be allowed?
Hmm, it seems difficult to use "popularna" for the translation of "Who did it?" ;-) But for our sentence, I'd suggest "Która z nas/was/nich jest bardziej popularna?"