If you're above 50% and Dedede grabs you, it's a stock for most players. LEARN TO MASH. You can increase your survivability to 100%+.
Basically stagewise you want a very large stage with platforms. Don't fool yourself into Castle Siege however, you will get ***** there. Also DO NOT GO TO RAINBOW CRUISE. You will regret it. A good option would be the Pokémon Stadiums, though know that most Dededes know them pretty well. It's going to be a very, very long match. If you can you should force the match to time if you have a lead. A lead is something that will be incredibly hard for you to get.
Norfair and Jungle Japes are also good stages. In general, any stage which forces Dedede into the air is good, with the exception of Rainbow Cruise. Just make sure you actually know the stage.
Your ban depends on your stagelist. On a conservative stagelist, FD is probably your ban. On a very large one, with 2 bans, You want to get rid of Green Greens, FD, or Onett, depending on just how crazy the stagelist is. You CANNOT beat a Dedede who knows the stage on Onett, so if for whatever reason it's legal make sure you ban it.
DAir is incredible on shield in this MU, as most of the time we won't have a full one. If you manage to surprise us with it, you can hit us with the grounded hitbox. This gets us into the air. YOU WANT US ABOVE YOU. You are Mario. You have exactly one thing which could be called "good". That is your air game. Juggling Dedede can be really easy, but watch out for a fastfall to grab.
Remember that a single grab at low %s means you take 43% (this is DThrowx4 -> BThrow). If you can't mash, and we have some space on the stage, we WILL CG you across the stage, throwing in pummels, to start the infinite.
Our BAir outranges everything you have, and beats out FSmash. Learn to respect it.
Have FLUDD charged at all times. Learn to space your aerials against Super Jump coming down.
This MU is incredibly, incredibly hard. I rate it at -4 for Mario, or 90:10 Dedede's favour, whichever system you're using. WITHOUT the infinite, I'd rate it as -4 for Mario, or 75:25 Dedede's favour.
Honestly, your best bet is to find a secondary.