IMHO the problem isn’t with the concept of avatars, which I think add a nice touch of customized visual appeal, but with posters who cram too much detail into them which then gets lost in the thumbnail size. It often works best if you use a simple drawing or symbol rather than an actual picture.
There’s a good thread here on the evolution of an avatar, in which some of the issues with that thread’s OP’s original ideas are discussed. The OP eventually came up with a pretty good one, and then @BigT stepped in and edited the image to make it bolder, and added colour. In the end, @Senegoid ended up with a very distinctive and recognizable avatar.