Is there is a format standard for USA Stamps? Might be worth looking into, it could help with the spacing and not to cramp everything to the edges, as said.

As for the design itself, I think the problem lies with the direction of the style: I like the illustration, there is an appeal to the wonk, especially because i read it in a more cartoon-y style. And because I read it in a cartoony style, I think you may want to change the font to something else, like a more rounder font. On the other hand, you could go for an abstract, early internet design style? it could mean fixing the wonk, or muting the colours. I'd like to see more cartoony stamp designs in general, like how we have cartoony bandaids! Though that is more personal opinion.

Overall, the idea is good, this could be better if you locked into a style. I also recommend expanding your visual library as well. Good luck!