Two pieces of advice:
1. Keep practicing. You'll get there.
2. If anything, Mario has a healthy meta in Ultimate. This is especially evident where he was one of the most used characters out of the top 128 seeds at Genesis 6. Zenyou and Dark Wizzy are especially doing great with excelling in the meta and making Mario all the greater, and Teb and Rice are doing pretty alright as well, even though Zenyou and Wizzy are on a different level.
The reason why you probably see him as "trash" in Ultimate (when really he's got good tournament results right now) is because he is HARDER to play in Ultimate than in Smash 4. His combos are just as good (if not better) but they are harder to pull off because you can't do grab --> d-throw --> u-tilt and u-air --> profit like in Smash 4. In Ultimate, his up air strings are still very solid, and SJP is a much more reliable kill option. However, the set up to pull off such a great combo is harder to set up because Mario's grab game is nerfed. However, his combo game was slightly buffed to compensate.
Besides, his recovery is fine. It's simple and straightforward, but it functions. It is possible to pull off his combos but you need to set them up appropriately and follow-through appropriately. And, sword characters always give him issues because of his weak range, but he can get around that with smart play (and it helps that his frame data is excellent).
Finally, with focusing on the phrase "I don't see why anyone should play Mario in Ultimate"...First of all, Mario may be tough to master but he's easy to pick up. He's straightforward and user-friendly. Secondly, his combo game is very impressive once you get a handle on it. Thirdly, his kit has more utility than most characters in the game, and because of the sheer amount of options he has, it makes him a popular character in the meta right now.
Try watching this. Shulk is one of Mario's hardest matchups in Smash Ultimate (IMO), and yet Zenyou is able to take out the best Shulk main in the world right now: Nicko...and 3-0 too.
