"Ugly"? I thought fans pretty much all loved the BoP.
I think it's an eyesore. Really, I don't much care for any Klingon ship designs aside from Jefferies's D7 and its very faithful K'tinga
counterpart (which is virtually identical in shape, just more detailed). Jefferies's design looks like a dragon in flight, elegantly menacing. The Bird of Prey was consciously designed to suggest a football linebacker when seen from the front -- just bulk and brute force, all the elegance lost. Not to mention that it doesn't really work as a Trek-universe design because it doesn't have any recognizable engine nacelles. (Well, okay, we've seen more nacelle-less warp ships since then, but none of them were Klingon.) And all the TNG-era Klingon ships were blocky and angular and covered with kitbashy surface detail, and the sleek simplicity and grace of Jefferies's design was long gone.