I'm glad you made it onto Equestria Daily, and this looks really great—but! It's not perfect. And that's what I'm here to overexamine and tear to shreds.
There's two things I'm critiquing here, the art and the joke. I'll begin with the joke.
The punchline is a little funny, but there's two jokes going on here, and they both want the limelight. There's the good ol' "that's what she said" joke, and this seems to command the comic up until it hits. Then the Molestia gag comes in and hijacks the wheel, throwing off the pacing and the balance. I think you should make only one of them the primary joke, either axing one entirely or making one just a support. To make the "that's what she said" joke the primary focus, the the punchline should be the last panel. Molestia, if she's there at all, should be relegated to an easter-egg style cameo at most.
If you prefer the Molestia aspect, put way less weight on the buildup of the "that's what she said." Finish it quickly, maybe even in the very first panel, before having Twilight ask who "she" is and Molestia walks in. You can choose one or the other, but you can't have both in there, vying for attention.
You also have a lot of buildup that feels somewhat disconnected. You have Twilight mention the extremely specific measurements of the dowel, then she just complains that it's "wrong." I think your humor could be helped if she complains that it's wrong by announcing its incorrect—and preferably similar-sounding—measurements, to which Rainbow asks "what's that mean?" or something to that effect.
Panel 1 and panel 2 could probably be combined into one, since the dialogue in the first isn't really necessary at all.
The insertion of the "one job" meme feels shoehorned, distracting and, dare I say, tired. Try to avoid sticking little allusions in here that could distract from the primary purpose of the joke.
If you're going to use this much setup at all, you probably want to mention more elements of the punchline before it happens. Take Stephan Pastis' Pearls Before Swine. In strips like this one (the title is a link), it spends almost the entire time setting up every element of the pun, whereas the only setup you make for the "Uranus" gag is passing mention to the telescope. Since "that's what she said" jokes are more or less puns, it would help to have the same kind of setup that Pastis does.
Now, the art frontier. In general I like it, that doesn't mean I don't have some bones to pick. To start off with, everything here looks like a trace from a show pose. I'd encourage you to improvise and come up with your own postures and angles, rather than copying what's already established and fairly well-recognized.
What's up with Rainbow's cleft upper lip? It may happen on a cat, but cats aren't horses. Their lips are single strips like ours.
Molestia's smile looks a little stiff, same with her eye. Make her pupil smaller and her smile wider and more curved to up the creep factor.
In the third panel, Twilight's hair is very high, and that makes her head look like the one on Frankenstein's monster. Shift it down so it sits more logically on her ball-head.
Not a fan of the enormous space you've relegated to your watermark, which most artists only have a small black strip for. It's overly commanding and a little obnoxious.
Well, sorry if I was too blunt in here, but you did ask for a critiques in here. Hope I was able to help any way, and if you have any questions you can send me a note or something! Can you comment on critiques? I don't think you can...