Are they ever rejected for not being funny?
Yes, one in a while. Which says a lot because I let some pretty dumb comments through. If I think of it next time I'll post them here - the lack of funny in some of them is actually funny.
And almost anything that is written in l33t, even partially, is thrown out. If people can't take the 2 nanoseconds extra that it takes to type out "you" instead of "u" they shouldn't own computers.
Likewise, anyone who takes the time to type out a comment consisting of "lol," "that's funny," or "omg" or variants thereof shouldn't bother in the first place.