>>151154543 (OP)
I can see it actually.
Mandy would find Mac's intelligence and creativity refreshing, while Mac would be fascinated by Mandy's macabre nature, probably challenging himself to get her to smile.
People like to ship Mandy and Billy, but Billy is genuinely just too retarded for Mandy to see him as anything other than a goon for her schemes.
>>151154543 (OP)
I imagine Mac wouldn't be scared of her, but he wouldn't tolerate her rudeness either. Eventually she'd say or do something that crosses the line for him and he'd politely but sadly tell her that they can't hang out anymore. Which would break Mandy's heart and remind her why she only hangs out with Billy and Grim.
>>151156199
That's how you make the ship work: the second half of the episode is Mandy trying to figure out how to apologize to Mac (something her body physically rejects.) You could get some cute stuff out of Mandy genuinely trying to be friends with someone and struggling, and the other person eventually understanding and helping.