He was an awful singer for every other band BUT Smashing Pumpkins. But that band, with that sound? He's amazing and you would have to be a fool to deny it.
Sure, there are some singer that could cross bands and genres with ease that are FAR better. Freddy Mercury, for example, could have headlined any of a dozen or so bands with his range. Danny Filth (god I cant believe I am admitting this) has an AMAZING singing voice when he isnt doing his bullshit black death suicide goth gayness sound. Theres probably a dozen or so others, bit I'm drunk so fuck off. (Oh shit, Corey Taylor)
But then there are one-sound singers that can only make it work with their ONE band. Look at Fred Durst, he is an AWFUL singer overall. But god dammit, you put him with his band and his sound and he can blow the fucking roof off. Weird Al, he perfected his... thing and there isnt a major musician on the planet that doesnt respect him. I saw Neil Young mentioned earlier, and yah his overall vocal skill isnt that great. But fuck if he didn't write songs that took advantage of it and made his mark on the musical world.
In the end it's all about how you sound with your music, not without. Corgan sounds fucking amazing with Pumpkins, and that's all that matters.