Hey there!
Since I'm also using a mouse for my images, I can say from experience that the curve tool provided by most programs is the go-to option for preventing those wobbly outlines, though I am not sure whether this is your intention seeing your thread title^^. Other than that you could perhaps give the shading a second look, for example with Litwick's fire being a light source. With round objects its generally a good idea to add shading on one side to give them a more three-dimensional feel.
If sketching out a draft with the mouse gives you problems, a scanned in image can bring good results as reference for your lineart. That's important if you decide to tackle more complicated subjects and poses, so don't shy away from those! :D
What I really enjoy about your drawings are the facial expressions you gave these guys, especially your Cleffa which virtually seems to come alive with that mischievous grin :> Makes for such a sweet image with that hat!