Marth can fan fireballs with his sword (Fair, Uair if necessary). He can hit Mario with rising Dancing Blade, which will give him INCREASED upward momentum if caped (I think). He can use Uair which will hit either way. If caping is your primary guarding tool, Marth can Bair sometimes. Mario cannot bet on Marth Bairing consistantly, because it's too risky since Marth may fair.
I don't know how potent Fludd is on Marth. I've never actually been gimped by it, but I think we can fan some of it, lol.
Marth's onstage game keeps Mario out, so Mario has a hard time using his speed to deal much damage to Marth. He can't approach from the air, and his whole moveset except his highly unsafe and telegraphed Fsmash is outranged by Marth's dtilt. Fireballs can be perfect shielded, after Fair which auto-cancels, and dtilt which is IASA frame 21 I believe. PS takes only 2 frames I believe, so it really doesn't provide ANY opening whatsoever. If I can approach Falco from the ground through a laser stream by walking forward and hitting L, R, or X (my shield buttons) with good timing, I am sure not going to let two fireballs slow me down. Mario has ZERO reliable ways to get into Marth; Fair and dilt with the occasional ftilt shut down ALL of his options, and Dancing Blade punishes too good. Retreating into the shield is awesome for Marth, because retreating Fair OOS or Dolphin Slash, or even Shield Drop Dancing Blade are all good options.
Also, I'm not offended by the vid, I just want to emphasis the fact that the Marth in that vid is not at the level that should be in this discussion. Your Mario is good though. Nice footstool. I would've appreciated it more if I wasn't yelling at the Marth for failing. I get angry at bad Marth players, lol.