For Mario, ledge trapping is always difficult. Mario is the really bad at ledge trapping, he doesn't have much moves that covers multiple options. His down-smash covers 2 options, so does his up-smash. And that's about it. It's usually best to go for a hard read, then quickly return to neutral. Holding shield in-between the roll getup and neutral getup is a also a really good option. Then play reaction and punish whatever option Snake chooses. Choose one of those, but remember the options Snake chooses and the options Snake chooses to counter play your options, to then go for a punish.
Mario