Yes, it is intended to be mocking. But no, it’s not about intelligence or the ability to spell.

Within US culture there are those who find something so objectionable that they assume that simply naming it will result in shared revulsion. Usually this is accompanied by placing emphasis on the word. “Teh Ghey” sort of evolved as a phonetic representation of this horrified emphasis.

Also, including “Teh” as part of the phrase suggests that those who see homosexuality as objectively objectionable also do not see it as one’s sexual orientation but rather as some sort of separate entity. They objectify Gay without any recognition of the men and women who are gay. Gay is not an adjective, to them, but rather Gay is a noun, like The Nazis or The Plague.

Actually, few anti-gays use the term “the gay” on its own, preferring “the gay agenda” or “the gay lobby.” It is, however, more amusing to truncate it to their real objection, “Teh Ghey!!”