And now I don't understand why I don't see badges like "Top MIRI donator" beside usernames around here. Was the idea not thought before, or is it more like I am missing something important here?

It raises the complexity of making a decision to donate.

Apart from that this forum doesn't use avatar to let people concentrate on content and not on the person who says something. The forum is not supposed to be a popularity contest.

There's Giving What We Can if you want to be a member of a "League of Extraordinarily Virtuous People". You can also put the fact that you took the pledge in your LW profile.