It actually is a really cool cartoony design, I think though that it is a bit bland for my taste. What can be improved is maybe instead of just regular color try to add patterns so it does not seem so plain.
I’ve added some markings to your design to help with understanding my critiques:
The blue “Trade Request” button does not completely fill the space with the blue background color. Also, the button is not aligned in the center of the x-axis, but is not perfectly aligned on the left side, either;
The “username” buttons are completely touching the top of the frame, which, on its own, is not visually appealing. In addition, the “cash” buttons are spaced away from the bottom of the frame, creating inconsistency;
The “Accept” and “Deny” buttons are not equally spaced from the center frame. In addition, the text in said buttons are positioned a bit higher than they should be. It also appears as though they are not perfectly aligned with the center of the design, either.
Finally, as a personal opinion, I feel as though the black borders are a bit thick on the UI. I also find it odd that the grey buttons in the center lack a black border, which creates a bit of additional inconsistency.
These are somewhat tiny details, but when it comes to design, little details can make a big difference in terms of presentation.
I like it, for your first UI, you really did good! However, I’d just add some shadows to make it looks better. For exemple, the “Trade Request” button, you can add a slight blue frame at the bottom, just up the black border which would be a darker blue and it would make a pretty good shadow, for not much effort and I think it would be pretty amazing if you would do this to all of your buttons.