Although a tablet may help, you shouldn't blame your faults on a mouse.
Try using layers so your colors don't color over the lineart. Spending more time on art also allows you to make better pieces. In addition, you may add shading too add a bit of realism or depth. If you have any questions please memo me. I realize I'm not an amazing artist, but I can provide tips if you need them.

Also, try drawing with the pencil, airbrush, or pen (if you're using Oekaki Shi Painter) tools.