Fireballs. What's the deal? I read your post about you getting to throw them in all opportunities against this opponent because mindgames but it was a poor usage of them in a lot of instances, let alone a poor usage of an opportunity to better understand your opponent. You tossed them out without really looking for your opponents reactions to them, or following them to capitalize. When they did result in an opening, it seemed like it was by happenstance of their approach versus your default strategy, which is to say, you sat there plipping fireballs while your opponent approached, he would accidentally bump in to one, apologize politely to the orb of flame, and then be promptly punched in the face by a surly plummer. It's not necessarily an awful strategy. It put you on top of most of your exchanges, BUT it was predictable, and not optimized. Fireballs are an AMAZING tool. But your use of them seemed so crass. They're a nice way to open your opponent up, needle them, make them swear revenge upon their slightly charred faces. But you just put them out there without gauging the opponents reaction, without following them like your nubile children (so full of potential), without an idea of when it would be considered a bad time to do it (point blank range, with an anvil that reads "10 TONS" plummeting towards Mario's unprotected dome). Use them for what they're useful for. They'll yield MUCH better results.
Playstyle. A lot of things fall into this "category" so it gives me a nice way to tie parts of your game together, and summarize what I think needs work. My main gripe really falls into your playstyle, in that I feel that you play Mario like a pseudo Falcon. From watching your games you seem to weave in and out of your opponents space while pressing against them with powerful moves or combo starters, and making them not know what to expect with fireballs and cape. This isn't what Mario does. Mario is a wall, or fortress, if you will. His intention (gameplan) is to work his way into his opponents space and exist. It seems strange but it's a very basic way of outlining what he wants to do. He can't pressure as well as other characters, or zone, or mixup/ psychout, or anything really. But he fills a position that goes overlooked in most games, and really shines in Project: M, which is the Jack of All Trades. He does a lot of things pretty well, which lets him fill a roll that most characters can't fill by being very reactive, and punishing opponents for all their faults. He wants to push at them, working his way into their space, their livelihood, and then set up camp. So playing optimally, you're weaving in and out of your opponents comfortable space, taking full advantage of all of their mistakes and attempts, while also being present enough to create discomfort that CAUSES mistakes. Be close to them, as much as possible.
Edgeguarding. Is really the last thing I feel I need to mention, and (thankfully) there's not much to mention here. I just felt you were using cape too much, in situations where it did you no good. Mario has much better options for this in most situations, once you account for the capes lag and predictability when spammed. While his offstage game isn't godlike, he still has a pretty good spike (Fair), an amazing swat that sends the opponent away at a wonderful gimping angle (Bair), and the ability to grab the ledge (to gimp). I know that these seem a bit obvious but from watching your matches I didn't see these tools employed in any great quantity, which was alarming to say the least considering they make up a good deal of Mario's kill potential, which is to swat away from the ledge and edgehog thusly.
I hope you consider employing the techniques I've outlined here, as I have faith in their ability to facilitate better play from you. As a last note, I would recommend you turn your attention to Nausicaa's post listed below, as it outlines some of the faults in your reasoning in your above post.