I wouldn't call it a failed drawing, though, you may be joking in the description. What I see you trying to accomplish is conveying is Derpy's carefree joy and energy and using that to tell a story.
You definitely portrayed this nature in Derpy with that dynamic pose. Bonus points on portraying that pose and maintaining correct anatomy. The selective use of detail makes her smile the first thing you see, followed by the dynamic pose, fully conveying her joyful energy. After establishing this aspect of her personality, the eyes are free to take in the rest of her pose, and move down to the TARDIS, noticing The Doctor's near-panic at her antics. A nice little story, if not very new, told rather skillfully.
The only thing I would suggest is adding a bit more detail to the Doctor. At this point, the eyes move down to him simply because there is more to see in the picture. Derpy commands our attention fully. A little more detail might help draw the eyes to him after having first seen Derpy.
: Ditzy, what're you doing out there?! :I believe I can fly! :That's not the point. There's no gravity or OXYGEN in space, and I can only extend the shields so far. : Derp. :Really, Ditzy...Common sense!
If someone had told me a minute ago that it would be possible to mix up MLP, Portal AND Doctor Who in just one picture, I would have laughed at him. Now I'm laughing my ass off. Great work! Btw: I hate false humility. Stop lurking for cheap "But no, you aren't a fail at all blabla"- compliments and stand up proud for your work.
"GAH! Ditzy! What are you doing OUT THERE?!" "What am I doing out here? What are you doing in there?" "I LIVE here! But that's not the point--" "Oh c'mon, Mr. Grumpy-Pants, get out here! The water's great!" "Th...bhu...gah....THERE IS NO WATER OUT THERE!"