Welcome to Tapastic! Your art is really cute. ^-^ If I had some advice I'd say maybe you should give upping the darkness of your shadows a try? Your drawings that have more contrast in the shadows definitely look the best!
(her It might feel weird at first but maybe do a drawing where you don't mind so much if you mess it up and give it a go, it would probably make a huge difference! (: (If you're currently using black as well, try using something like purple or another dark colour instead?) I'm only saying that however because you said you wouldn't mind some constructive criticism, and your art is super cute as it is! (:
If you still want characters to draw, maybe you could try drawing Rebecca! ^-^
(her ponytail changes shape a lot depending on my mood, but go for the bottom picture's shape for this )