Models like the Honda Hybrid Accord and Civic didn't do nearly as well because aside from the badge and carpool sticker, they didn't scream "I'M A HYBRID LOOK AT ME SAVING THE WORLD"
My hunch is that the Leaf was an attempt to capture some of that "conspicuous hybridization" momentum. Agreed, though, it's an ugly car.
I find the Prius OK, not beautiful, but not ugly either. Don't have one yet, but would likely consider one as a practical replacement vehicle.
I recall hearing somewhere that Nissan did for aerodynamic reasons to make sure the slipstream goes over the side-view mirrors, but boy does it look ugly. As an engineer, I usually never find myself arguing in favor of aesthetics over effectiveness, but in this case even I think they should have done something different.
That said, I'm not a car designer, so there might be other factors going on there.
I've driven one a few times. It's ugly, but not more ugly than a lot of other econo-cars like the Fuse. I'm tall and it's fairly comfortable inside, and it performs fine.
If I was still a daily driving commuter and still had two cars in the household, I'd definitely be interested in the Leaf because it would save me $250 in gas a month and let me drive in the carpool lane.