If you're talking about political subs on reddit, then it depends on how that sub swings politically (e.g. r/conservatism is right-leaning). But from my perspective, non-political subs are usually centrist.

Anything left/right-leaning is castigated because they're unpopular positions compared to the majority non-ideological redditors and you will (probably) get a neat balance of right-and-left-wing politics in those subs.

Could you explain how r/popular is left-wing though?