and why do they only spawn in certain seeds?

look in your minecraft.jar skinsyou will see mooshrrom and pigMAN

The concept of the Mooshroom till now sound like the stupidest thing ever, to have a mushroom biome and all seems great, but that cow just looks so bad... I believe they should be more inovative and creative mobs, with new looks and interesting twists like the silverfish and enderman, making a cow red with those things on it... is just blah... though I find interesting your guess, if it's really infected it might get some good twist on it, but still looks terrible, and if it attacks me I will laugh so hard, cause it looks like a clown and not scary at all.
With luck it will be one of those mobs that will never see the daylight, if not, all you guys gona be happy and blissful with your new friend :)