It's all about the mojo, baby. I don't think she's attractive by "normal" standards. And I couldn't care less. I'm glad they made the star of the show someone who isn't the standard comic cover superhero female. I like all sorts of strong females, but it's nice when they come in different ages, shapes, and sizes.
It's the same reason I'm sure many of us find our significant others sexy.
We like them as people, we like what they do. I'm sure we're all pretty average looking folks, ourselves.
And, anyway, we might as well extend this to all captains - why do the ladies find Sir Patrick Stewart or The Shat such beasts? Picard's an older balding gentleman, and Kirk's a bit portly with a ferret on his head, but they're still considered sexy. I would again venture that's it's allllll about their oodles of man mojo.
(And probably Picard's lovely voice.) Male characters sometimes get a bit more of a pass on this than females, too, by the by. These threads pop up every now and then, but why is there never a "How is The Doctor Sexy" thread? (He totally is, by the way.)
And, of course, YMMV! We can't all like the same things.