Peaks and valleys happen when practicing Smash. Plateaus do too. You just have to be patient and keep trying. If people are catching what you do in advance a lot, you're probably choosing a lot of the same options. This is a bad habit every player has, to an extent. For the next while, your goal when practicing should be to focus on mixing up your options. At ledge, try to think about all or at least some of the things you can do before deciding on an option. Remember that you have many: jump, neutral getup, getup attack, roll, drop down and do an up air before regrabbing, double jump aerial, and character-specific stuff as well. Remember that you do not have to pick a ledge option asap. Waiting a bit can get you back safely in the right scenario.
Remember also, that you have many mixup options in neutral. Mario's up B is a very good out-of-shield, but is risky because it has a lot of lag if you miss. You want to develop a good sense of using it only when it will hit. Mario has other good oos options. Neutral air is much safer than up B since it has pretty much no lag. Up smash will hit behind Mario if the opponent crosses up, and is good to look for at kill %. You can simply press down if you're shielding to go for a spot dodge, which you can do a lot out of. This is a good option if you think your opponent will grab. You can shield grab - shield grabs are of course pretty bad in Ultimate, but they're still an option. I often see high level players whiff their oos when they had an opportunity to shield grab instead, simply because it's so underutilized. People won't expect it! And there are the patient options, too. You can simply drop shield or roll instead of choosing a more standard option.
And this may sound silly, but oftentimes, one of the best things you can do is... Nothing. You can just sit there and wait for your opponent to do something, then react and punish. Let's say you jump, and you think your opponent will advance, so you down air. Your opponent didn't rush in, so now you're stuck in the lag of down air and, even if it is only a few frames, it's just enough time for your opponent to punish you. But if you jumped and did nothing? You would have no lag. You would be able to shield their attack. Here's another scenario: Your opponent is shielding. You come in with a neutral air. It hits their shield and you drift away a little. Neither player really gains or loses much. Fine? Sure. But if you had jumped and instead pressed nothing, you would have been able to Tomahawk, which is were you jump, land, then immediately grab. Mario, as well as most characters for that matter, gets a lot off of throw combos. Now, instead of hitting a neutral air on their shield which did practically nothing and reset neutral, you've done 40% off your combo!
Something you should also work on is looking at your character less. I know it might seem weird. You might think "Why wouldn't I look at Mario almost all the time? I'm playing him and need to know I'm doing things right." But, believe it or not, it is far more advantageous for you to look at your opponent's character as much as you can. That way, you can pick up on their habits and punish said habits. They air dodge after you combo them? After you see them do that a couple times, you'll know, and be able to sneak an extra hit in the next time. Muscle memory helps greatly with this. Once it becomes second-nature to do your combos, you don't need to look at your character to be certain they will work.
And lastly, patience is key. I once lost a match because I focused way too hard on trying to approach and trying to end the game then and there. I once won a match because I focused on just slowly doing damage over time and getting in the hits I could. Sometimes it takes a long time for you to find that KO, and that's okay. Sometimes you and your opponent spend a minute or two not really hitting each other, and that's okay. Stuff like that happens to top players frequently. Assuming you're playing with tournament rules, 7 minutes is a longer time than you may think. Remember that you have that time. If you start going for smash attacks a ton once your opponent is at 100, that's a bad idea because those are your most punishable moves. Smash attacks are best saved for when you're sure they'll hit, and reads. If you're throwing out fire balls and neutral airs when your opponent is at 100, sure, those won't kill. But what they will do, is tack on even more %. Now your opponent is at 160 near the ledge and is shielding because you've thrown so many fire balls out. You roll behind and seal the stock with a back throw. This isn't how every stock will go, certainly not. There will be opportunities to close out stocks early with smash attacks. They key is being able to recognize those scenarios, know that they are rare, and keeping a cool, patient head the rest of the time.
Hope this helps! I know it's a lot, but Smash is a very deep game.
