I'm no expert on the avatar side of things, but I'll try my best.
Only thik I see wrong with these is that you're trying to use textures and brushes (vector) over the focal points, which is really clear in the Nami and Robin avatars, and they don't really look too good. You're trying to focus everyone there, you don't want to be trying to cover it with nonsense.
Also I believe it looks like you've used the background colours wrongly in my eyes. Your Name for example is a big red background, and red is quite a strong colour, and it seems to be dominating over Nami. Robin is the same in the retrospect of the purple, but it's not as bad as the Nami one. Zoro is probably your best in that regards, because you've used the darker green background with his lighter green colour scheme, and they compliment each other really well, and it's easy on the eyes.
So all in all, I believe that your Zoro is your best work, because of your colours, and you're not trying to obstruct the focal point either.
also marimo