Yeah, it's pretty well established that this match up is good for Mario. Dedede doesn't really have any non-read-based combos against a Mario with solid DI, while Mario's dash attack, nair, jab, down throw and up throw pretty much set up into a guaranteed follow up on Dedede until about 90%, and oftentimes higher if the D3 player in question doesn't have really good DI. Mario has a far better projectile, and as such doesn't have to approach Dedede unsafely, thus making Dedede's stronger zoning tools (big disjoints, crazy shield grab, aimable projectile) not all that useful in the matchup. His dash speed and wavedash are better, and he's overall a faster and less laggy character, with the exception of fall speed. His side b and and down bs' stall properties also give him good mix ups on DDD's shield which, combined with the king's already lackluster OOS options, make it really rough to get out of shield pressure and back into neutral once Mario spaces a fireball that dedede can't power-shield on reaction.
Additionally, cape+dedede's unsweetspottable recovery=easy gimps.
The one area dedede beats Mario is, as per the usual, offstage. However, Mario has a pretty unique and amazing recovery, that can be mixed up a lot if you have good DI and save your double jump. If you recover high, you can also usually throw out a fireball then airdodge back to the stage safely.
Honestly, the only reason it even seems close to even as a MU is that very few Mario players have any experience with the matchup.