People say that it's because it's ugly, but I don't think that's it, because lots of "ugly" things are aesthetically pleasing within the context of their world.
I think the problem with this design is that it's incoherent.
Like, WHAT am I looking at here? What is this design telling me about this character?
Ok, so she's got the pretty pink/purple dress of traditional femininity mixed with tights and sports shorts. Got it. You could do a character like that. A girl who is very feminine on the outside, but has a Tom Boy side, or loves sports/heavy physical activity, and is actually way tougher than she appears.
But then you have that Beaker-Head, which looks like it's from a completely different character. And she's got those jug ears that jut out from her tube-head in a very jarring way, and that nasty "beauty" mark that looks like a tick has embedded itself in her face, the lack of nose, those weird bruised lips and gopher teeth, and those beady little eyes. These are all hideous, of course, and none of them gel either with each other, or with the rest of the design.
But what really makes ZERO sense from a character design POV, are the giant nerd glasses, and the crew-cut. The glasses tell me she is either a) a nerd, b) too poor to afford fashionable glasses, or c) a socially awkward weirdo who doesn't know how to dress herself. But the character poses she's doing tell me that she's a brash, tough character with tons of confidence, so that doesn't work at all, and her outfit doesn't fit the nerd look, so she can't even be, "The nerdy character who turns out to be super tough."
Then there's the crew cut. Why a crew cut? It matches NOTHING from the rest of the character. Is is because she's supposed to be a tough girl who don't care about that kinda thing? Then why the earrings, glossy lips, beauty mark, and super-girly dress? Her design does not reflect a coherent understanding of what her character is supposed to be.