The ones that I've liked to play are Ike, Ness, Cloud, Jigglypuff, Bowser, Marth, Mario, Link, Ganondorf, Falco, Villager, Shulk, Corrin, DK, and Pac-Man.
Assuming you're not playing at top level tournaments:
Ike should be fine, I guess. Ness generally works pretty well until you run into his 40:60 MUs, then it's hard. Cloud, Marth, Corrin all work really well too. Cloud and Marth are stronger than Corrin but many people don't know how to deal with Corrin, and she's still a very strong character (though probably not top tier). Jigglypuff... I'm guessing it'd be hard to play her in tournaments. Ganondorf, on the other hand, can work pretty well at lower levels. At top level play he doesn't really work all that well, but at lower level play you can get more hard reads and win games that way.
Donkey Kong is very solid, as is Bowser. Mario is easy and strong, though most people should know the MU by now. Link is suprisingly potent if you know how to play him. Villager is a high tier and could probably work, although I guess it might not be the easiest character to play in tournaments. Shulk is solid but takes
a lot of work to do well with. Falco and Pac-Man are kind of meh but you could probably do well with them at locals.
But if I chose a medium or low tier should I have 2 mains? Just some help would be great!
Depends on what your local scene looks like. If no one plays your bad MUs then it's fine, and for 45:55 MUs you can probably just play your main anyway. If you have any 40:60 MUs or worse you might want a secondary, though they are still winable with your main.
So far the biggest stuff I struggle with is just not moving so fast and not being able to tilt properly.
Consider using tilt-stick (attack), it makes it easier to do retreating aerials as well.
I do know that I tend to have that habit of wanting to be good with a medium to low tier character just because everyone ends up being really good with the high tier characters but I should probably get out of that habit as well.
I think it's fine to have this mindset, low/mid tier players make tournaments more fun since they add diversity to the cast. Many high tiers are rare as well (such as Villager and Corrin*).
*Corrin is top tier, high tier, or high-mid tier, depending on who you ask.