I really don't think he even likes Christopher that much. I think the reason for friendship is twofold.
He needed a new sidekick after Mattgew left. Who else is he going to convince to neigh like a horse? (Maybe Zoe if she'd been in earlier)
Secondly, he suspects Christopher and him as a pair will appeal to people as a pair, but as the "funnier, more handsome and popular one" he'll soak up all Christopher's votes when he inevitably goes before the final, especially as Mark can use that for full sympathy effect.