This is a very cute, very neat picture. The craftsmanship and cleanness of the shading and line-art is wonderful, everything's very smooth and and gradual with the shading. Color scheme for Derpy Hooves/Ditzy Doo is great, nice use of the purple-yellow complementary color scheme there. However, color scheme for the picture as a whole is jumbled; you have your good complementary scheme, but then you have reds, pinks, greens and blues... if an object's color scheme can be altered (like the bowl and the bubble blower) and it won't break any specifics required for the piece, try and make it all fit. Also, their left leg seems a little off in the placement of the knee, it ought to be placed further down the leg where it's even. Other than that it's a very tidy piece, and very cute!
I'm not sure I can accurately critique this because I love MLP so much and I love Derpy, but I'll give it a whirl. XD
I like how you took the style of MLP and made it your own by throwing in some differences. It's simplistic, but you made it that much better by throwing in extra shading. It looks almost like claymation now instead of animation. The shaded detailed style on her body hair and the bubbles nicely contrasts with the sharp boundaries on the reflection of the bubbles and in her eyes, as well as the colored outline borders.
The only thing that you could probably fix is her right hoof. You should separate it from her body, in the fact that it looks off with no space of white between it and her belly, almost like it might be a flipper of some kind.
That's the only thing that I can see wrong with the entire thing through. It's brilliant artwork otherwise, and I'm a fan of the MLP style. ^^ Keep up the good work. :3