Since Lancelot has (Magnus is forced to admit) been demoted from 'worst guy here' for weeks, if not months, now, he says, "She likes you, you know. She wasn't sure of you at first but she thinks you're really... nice." And since Magnus does genuinely like being impertinent to authority figures even if he does think they're mostly okay, he makes a face, then, like wild, right?

(The fact that his own impression of Lancelot could be argued to have followed somewhat of a similar, if longer, trajectory clearly doesn't bear mentioning.)