I think they don't have to be - Sokka was on the council in the flashbacks - but I would suspect they currently are? The only societies I remember seeing in AtLA where the leaders weren't benders were either ones that apparently didn't have any (like the two feuding tribes whose names I forget) or the Southern Water tribe who'd had all of theirs killed off. Not sure if that was intentional on the part of the writers, but it does make benders seem a little more like they're an elite class.

It would make a lot of sense to me if non-benders were beginning to feel unrepresented by the current system of four representatives, one for each element, now that they're living in a city and they have identities that aren't as closely tied to being non-bender members of X social group. I'm quite positive they won't get into that on a children's show, but they could hint at it by having an Equalist political party develop?

I do at least think the show definitely realises that Korra's willingness to take advantage of being the Avatar and thinking of herself as special is something of a flaw, so it seems likely that that will be addressed at some point? AtLA was pretty good at demonstrating that Aang's role was generally supposed to be mediator rather than to pick a side.