I also found no sense in Alucard's appearance, since he was beautiful as Trevor and then became a monster, but Gabriel remained handsome when turned into a vampire (even when he was just newly turned into a vampire in LoS).
He took ALL of Laura's blood. In other words, he completely bypassed any and all stages of the chain, and went straight to full fledged high vampire.
I didn't know about that feeding explanation, but it seems a little far-fetched to me.
Given that Gabe looks like shit after not having fed in centuries, it makes more sense than you think. I mean, Assuming Dracul has no problem with feeding on blood, (he had completely embraced what he has become- he even got himself a fancy new outfit) he would retain his looks for as long as he feeds now and then. Alucard on the other hand, seems to have turned under different circumstances. For one, his change was not voluntary, unless by being dead it doesn't have to be, and he skipped all stages as well, skipping the monstrous man-bat phase.
Back to theory here, but maybe because he is of Gabe's blood he doesn't need to go through them? Either way, probably something to do with being dead, or because he didn't bypass ALL stages, and went to a close enough stage. (more corpse-looking, but not a bat man)