I don't understand if there's an inside joke or something I'm not getting, but right when Mario meets Mallow in Mushroom Kingdom when he starts crying after Croc steals his frog coin, when Mario approaches him and stops, Mallow goes, "There was SOME reason" (in reference to why he was crying) ... then Mario falls face forward to the ground, but it's never made crystal clear why Mario does that after he says that. Can anyone recall this part of the game? I don't get what's implied.
It's a Face Fault. A trope largely popularized in Anime/Manga and other Japanese media, although examples of it exist elsewhere.
It usually happens when a character is so shocked/startled by an absurd/non-sequitur/stupid remark or action of another character that it literally floors them.
In the specific case you mention, Crocko had stolen frog coin from Mallow - it belonged to Mallow's grandpa. An item so important to him that when Crocko gets away, Mallow cries to the point of causing a downpour. Yet seconds later when approached by Mario, he doesn't even recall why he was crying. The absurdity causes Mario to Face Fault.