If it has a canon answer, it's not primarily opinion-based. POB questions can't be answered with anything but opinion pieces and pure speculation. A canon answer is always better than that.
For example, would a question like 'who is stronger, Dumbledore or Gandalf' be on topic if there would be a canon based answer? (for example, if JKR addressed it in a tweet)
No, because this counts as a "Gorilla vs Shark" question, comparing two different canons with no real connection. SFF policy on Gorilla vs Shark is here. JKR addressing it in a tweet wouldn't be a canon answer either, because although her word is canon for Dumbledore, it isn't for Gandalf.
I have a specific question in mind based on an obscure piece of Supergirl canon, but wanted to know if it would be on topic.
That sounds like it wouldn't be a Gorilla vs Shark question, if it's just about Supergirl (unless it's something like "who would win in a fight between Supergirl and Hermione Granger"). If there is a canon answer, then the question would probably be on-topic. No guarantees though :-)