I think the concept of Seymour was awesome - a vindictive half-Guado like he is - and his storyline was great, his role in the plot great, all that. But then his damn outfit and his voice made him just.... eh. I think he gets knocked down a few notches in awesome-villain-ness by his bizarre appearance and voice.

It can be argued that Kuja and Sephiroth and such are bizarre looking too, but they're appearance is actually more toned down than Seymour's. Bizarre, yes. But not to the scale of Seymour's bizarre-ness.

Plus, Seymour's outfit always gave the illusion of a pot-belly.