Actually, I suspect most spelling mistakes are probably the result of time constraints or lack of proof reading, not level of intelligence, but whatever fits to their silly stereotypes.
*and I'm saying this as someone who is generally pretty accurate with spelling to the point my other half uses me as a walking dictionary when he's writing something* (don't judge my spelling here btw

I do both the things I mention above, besides seeing posting on a forum as a very casual vehicle for communication)