Uh... most starters are pretty detailed. I think the main problem with it is that its body is just a single shade of green, so it looks a bit plain. Although it's similar to Chikorita in that respect, Chikorita has a few details to break up the plainness - for example, its eyes are red instead of green, it has a string of buds around its neck, and it has white toes. By contrast, Mimibud's details are on its head and tail, leaving the rest of the body looking plain. Maybe you could give it little claws and a pattern on its belly or something?
I had the same problem when I designed my starters. For example, Dipleaf's old design was a bit like your Mimibud design; when I saw Mimibud's sprite, it reminded me of my old Dipleaf sprite. If I can add detail to my starters and make them look better, you should be able to too :)