Random Matchups Dump:
Fox: 80-20. He beats you in every category. You can edgeguard him okay, and tech chases and combos aren't awful, but you'll never keep up in damage with fox. Plus you're losing neutral, too.
Marth: 65-35. Marth dominates neutral with superior range and about equal speed. Like fox, he will punish harder, often capable of taking a full stock from one opening. He edgeguards YL better than anyone else.
Falco: 65-35. Slower of the spacies, but still about equally fast as YL. Because falco is slower than fox, you are more capable of pulling bombs in this matchup. Even still, falco should dominate the neutral and punish harder than YL.
Sheik: 75-25. Only saving grace for YL is that sheik doesn't rush you down as much as the other top tiers. This can give you time to throw boomerang and pull bombs, but thats where the fun ends. Her punish game is unmatched, with chaingrabs and plenty of non-chaingrab options. And really, the neutral game isn't good at all for YL; sheik has plenty of tools to evade and punish YL's bombs and her normals are better. Oh yeah, and she can CC grab all of your aerials besides dair.
Peach: 55-45. A breath of fresh air coming after the above characters. YL doesn't win, but he definitely has tools to threaten peach. Their downb are very similar, but YL's is better in all ways except being on a timer. He can keep up in normals, but peach tends to win in edgeguards and damage racking combos. Once the peach has learned the projectile game, it can be difficult to combo into necessary dairs.
Puff: 55-45. Similar to the peach matchup. It is definitely possible, puff's slowness allows you to set your projectile game up freely. If you don't win neutral, though, you're gonna lose because puff punishes much harder.
Falcon: 60-40. The biggest problem with handling falcon is his speed. In a straight fight, there are plently of advantages YL has. Nair is effective in stuffing most of falcon's game, and YL can edgeguard him as well as any top tier. Even YL's stubby sword is disjointed enough to beat falcon's hitboxes. A good falcon will not fight you head on and instead out maneuver you on the stage, force you to attack defensively with his position, and then punish your lag with a strong combo. Against bad falcon's this matchup feels even, but against a good falcon it could be as bad as 80-20.
ICs: ??-??. Not really sure. Axe chooses this over pikachu often, which leads me to believe its a decent matchup. However, in practice I don't see much YL has going for him. On FoD you can air camp with bair->dj->waveland on platform, but on most stages you can't reach the platform with your DJ. Bombs are good at splitting and initiating a punish, but pulling a bomb can be dangerous. Sure, they can't wobble you when you have a bomb in hand, but they can still f/dsmash you. None of YL's ground moves are safe on shield, and most can be CC'd. His aerials can also be CC'd. Being frame safe on shield is important in this matchup, and YL struggles to be safe here without relying on spacing. I'm lead to believe this matchup is between 55-45 and 60-40.
Samus: 55-45. Slow and projectile heavy, YL tends to end up in a camp war in this matchup. Samus can go tit for tat, so its not free. As mentioned before, YLs stuff is very CC weak, and Samus is great at CC. Her normals, especially ground moves, outrange YL's. I've always enjoyed this matchup, but I can see an in-your-face Samus giving YL a lot of trouble.
Pikachu: 50-50. I don't think pikachu has many threats on YL outside the obvious usmash. He struggles to link it on YL, too, relying on weak nair or a techchase (or randomly throw it in neutral). Instead, pikachu often kills with nairs and edgeguards. Ground speed, these two are evenly matched. Normals vs normals YL usually wins, and I think YL wins the projectile war too. Pika does a little better with comboing and does well edgeguarding YL.
Yoshi: ??-??. No knowledge at all. I speculate that a yoshi of aMSa's quality would have little trouble countering the projectile game. Only plus is that bomb can't be parried. YL shouldn't have many combos, and won't be able to break yoshi's heavy armor until after mid percents (which means chip damage only and avoiding yoshi's counter attacks). I think it would be yoshi favored, 55-45 or 60-40.
Doc: 60-40. He has chaingrabs on you. He has dthrow->fair KO. His projectile is kinda decent, but YL's are better. He has lots of good normals and YL's nair won't get away free. Oh yeah, doc has a great CC game which is bad news for YL. If you camp like a mofo and chip him a ton of damage, you possibly can win. Straight fighting, YL gets bodied.
Luigi: 60-40. Wavedash forward is faster than falcon or fox dashing at you. Boomerang is virtually useless, since he can shield it then WD OoS dsmash before you can defend. Luigi is heavy and floaty, with poor aerial mobility, so YL can do some okay combos and finish with dair. But Luigi's high priority hitboxes give YL tons of trouble when combined with the quick movement of his wavedash. Luigi is surprisingly good at edgeguarding YL too, his dair is excellent at kicking your head when you upb.
Ganon: 50-50. Ganon hits hard, but YL has the speed to out maneuver him and the tools to kill him. As YL, your goal is simply to setup an edgeguard and dsmash/nair him to death. You can often kill him with an edgeguard before your dair would KO off the top. Both characters mess each other up, and find this matchup a welcome release from the woes of fighting top tiers.
Mario: 55-45. Easier than doc, but you still get CC'd and chaingrabbed. No dthrow->fair KOs, and mario's dsmash and bair don't hurt as bad as Docs. Definitely winnable, but Mario has a slight edge still.
YL wins every matchup below here.