Ok, so an answer to your question......... (other than learning the basics of how to play) I actually don't think playing online IN GENERAL helps you get better at the game and that's for one reason:
We're going to talk a little bit about input lag. Your buttons just take so much longer to cause an action to happen on screen that it makes any kind of skill (other than just a good neutral game) completely irrelevant online. Examples of this are pressing "up b" but for some reason online eats your input and you fall to your death or the up b comes out way later than you pressed it and you fall to your death, random air doges or rolls because the buffer system kicks in when you didn't actually buffer anything (due to input lag), moves not being punishable like ganon's down b because of input lag, combos either connecting when they shouldn't or not connecting when they should, etc (this list could go on for days).
Now this is NOT the lag of people with bad connections where the game randomly freezes for 2 seconds every 10 seconds or so. I'm not even talking about that, I'm strickly talking about the latency of your inputs online. So you have a player like Leo, who has it imprinted in his brain from all the practice and tourneys when to tech something, miss 3-4 techs and lose a set because he's flat out not used to it. Even players that ARE used to it miss techs online on a regular basis. I always describe teching online like winning the powerball.
Honestly it's unacceptable. Nintendo created the WORST online gaming experience in Smash Ultimate that I've EVER had the pleasure of playing from any gaming company out there (even EA). Then they have the gall to charge us for it. It's awful. And don't even get me started on GSP.
Ok, rant over.
If you want an experience that is a bit better (imho) it's the online tournament mode (not the casual one with items but the pro tournament mode). You don't lose anything by losing other than just moving on to the next tournament and the opponents get progressively harder as you win. Each tournament mode is active for 2 days at a time (I have NO idea why they don't just have both activated ALL the time but........ whatever I guess) so if you catch the casual mode just wait for a day or two and the pro mode will be active for the next 2 days. One thing to note is it's best out of one game and it's always FD... so it's not TRULY a pro tournament but it helps you not have to deal with QP.
As far as "getting better" is concerned I've trained a few people in my town on how to play. First thing I do is put them on Lucina. Yes... a boring swordie. The reason for this is you HAVE to learn the basics of the game to win with her. She's ALL about neutral game and winning the stage by skill. Learn her "A" moves FIRST (just in training mode). Learn how fast her jab comes out, learn her tilts and how to do them EVERY time you need them (put them on C stick if you have to), learn her aerials and what percents they kill at, learn her smash attacks, learn how to short hop nair, etc. THEN AFTER you learn her "A" moves learn her "B" moves. Learn her recovery, learn her counter, learn her shield break, learn her side B (all versions of it and how to do them).
Then go into QP and SPAM one game after another (don't worry about losing), you're learning HOW TO PLAY THE GAME. Lucina teaches you the essentials you need to GET BETTER. Keep winning with her until you hit the 4-5M GSP range and then switch to whatever character you like. You'll have a much better handle on the game then.
Also, once you get a good grasp on the mechanics and moves of your particular character look at these two documents for what moves you need to use to punish things. I'm a Zelda main so: my jab comes out frame 4 and is great against small characters, my bair and fair out of shield come out frame 9, my up b out of shield comes out frame 6, my ftilt (which is the best in the game) comes out frame 12... etc.
Smash Ultimate Out Of Shield
Smash Ultimate General Frame Data
In all learning this game is a process. If you have any questions feel free to ask me.