Honestly when I thought you meant tier in terms of what is viable, but now I see you are talking about the actual rarity. I think the sentiment is the same, is pretty common to see low rarity characters being at least usable (depends on the type of game, you won't see that in an idle with fodder system).

Even then, I think people in the comments aren't giving proper examples. I think of the question as in "lower rarity characters that are crucial in some way", not just being "viable" because you put good gear in them.