The picture is okay,but here's the many problems. The first one is, the cloud doesn't look like it just shot lightning. Maybe put a hard-to-see-but-there layer of lightning at the bottom of the cloud? And here's the the second problem. Twilight's mane wouldn't really do that, it would slightly poof up, instead of it looking ripped off from her mane. If it was ripped off from her mane, there would be little pieces of hair on her and on the ground. And, wouldn't there be more clouds and pegasi in the background? And is it just me, or is Twilight floating? Plus, I'd think that Derpy would be a little bit higher up, so she wouldn't know who she hit.
And now, i'm gonna tell you my voting reasons, if you don't mind me blabbering on.
Vision: 4.5. Probably the most reasonable I could give.
Originality: 4.5. You missed half a point, because of the title.
Technique: 5. The way you do your pictures are adorable, anyone can tell you that.
Impact: 4.5. Again, probably the most reasonable I could give.
Yet again, thank you for dealing with my rambling. I hope I didn't bore you.
Adorable, as always. The primary thing that I think could be better is that I think this particular image would've worked best as a three panel comic. One panel for Derpy bouncing on the cloud (or otherwise messing with it) with Twilight below, one of Twilight getting zapped, and this one. The lead in would make this panel more amusing, I feel, and provide some general cuteness in the form of Derpy messing with a cloud--and who doesn't love Derpy cuteness?
Outside of my thoughts on making it a comic rather than a single image, your work is always incredibly adorable. It doesn't exactly line up with the show's style, but that's very clearly not a matter of skill--you've got your own style here, and it's one that's really VERY compatible with brightly colored ponies. Both your usual style (as seen here) and your chibi style work well, but I think you made the right choice here in going with the non-chibi.
One thing I particularly like is Twilight's still dazed/startled expression. This piece definitely has a home in my folder of pony art.
Even in a drawfiend, I can tell what art is yours. Must be great to have a style that can be identified as your own. Though I do wonder if I would be able to tell if an art piece by you with a different subject matter could still be identifiable with you.