A f**king Genius

But than is a preposition not a conjunction in this sentence so the subject pronoun is slightly more correct because of the comparative nature of the sentence. For the most part the object pronoun is also accepted in general conversation. However, accepting it as is wouldn’t have fit the spirit of the joke.

"I" can be used in formal situations. Funsubstance isn't a journal, a business letter, or a dissertation. For this comparison "me" is the perfect option.

I acknowledged that already. The whole impetus behind my original comment was @rosalinas saying that other children had done better than she had. Correcting her was a cheeky way to imply that grammar was one of the subjects in which she had been surpassed. Complete accuracy had to be sacrificed for the humor to work.

Look I hate to be pedantic, but I have to be. You said that your correction was "slightly more correct", but it's not. It's only more correct in the situations that I outlined. There are many more situations (not only general conversation, which is the only one that you mentioned) where it is correct. I just don't want any non-native English speakers to get confused by an offhand joke. Now that I have completely killed your joke, can anyone tell me how to change my name to "jokekiller"?

Nah, this is way funnier.