First and foremost, because you recently started out playing you shouldn't have a secondary at all. It is definitely the best to focus on one character (your main) and to learn the matchup with them, instead of spreading the time between characters. Some characters for example have different jump squat timings which makes wavedashing between them slightly different and it takes a while to get use to it. There's also learning what your character can and can't do, what will get you killed, limits of their recovery, spacing, etc. Not to mention, you play other Smash titles which makes transitioning between the games tough to do.
Regardless if you choose a low-tier, mid-tier or even a top-tier character, you're going to fight high/ top-tier characters almost all the time. At tournaments, you will rarely see people play characters worse than Peach or Yoshi (varies) and only low-tier characters if someone is bored or trolling. Young Link being low tier means he has a tougher time against those better characters which will make the road to getting better much longer and frustrating. Though he has strengths of his own, it takes a while to learn what he can do and how he fares against all other characters (A.K.A. the matchup). If you were to use Mario as a secondary, which as said before I don't advise, it will take much longer to become a better player overall.
I don't recommend using Young link as your first main competitively, but you can still do it. Sure, there is debate about if he is a viable character or not and people say you should play characters you feel are fun, going into the competitive scene is tedious and grindy, like a Korean RPG.
TL;DR: not recommended, but if you stick with Y. Link, drop Mario as a secondary and focus on Link. Melee is a bit harsh on the characters that aren't too good as the game is pretty unbalanced but it is still fun and manageable. Especially if you get good and fare well in the meta which is developed quite a lot.
