MK has to respect mac in neutral, but MK gets silly advantage and momentum off one dash attack. DA --> U-Smash --> Punish his landing --> Convert into edgeguard.
This is only on FD though on platforms you don't need to engage him, if things get rough just jump on the platform and take a breather. If you get the lead you just camp him out, unlike Kirby and D3 Mk has good mobility ontop of multiple jumps ONTOP of his Cape not doing that silly lag instal glitch anymore.
If its a day 1 MK you scrapped from the barrel you will struggle against that Mac i'm afraid, go MM and cancer him.