I'll necro post because I feel this so hard. Like others have said best thing to do is just quit playing especially when you feel like you lost to some BS. I'm trying to get Shiek into Elite Smash now....LOL, highest I got her is 4.7 mill and then I drop back down to 4 mill and repeat, every day for the last few weeks and it's a nightmare. The people I lose to are typically terrible players who just spam. I played a Young Link who would boomerang then fire arrow, then dash grab as soon as possible and he landed with D-Air literally every time.
The problem is that boomerang has very little end lag, arrow even with zero charge goes across most of the stage, dash grab even when whiffed is difficult to punish and even D-Air has a huge hitbox and is super safe, and it kills pretty well. Even though it was close I lost and I just thought wow this dude is terrible and I still lost to him. It's pretty demoralizing obviously, IDK who these players are they seem like really scummy people and most of them will never rematch you so you can't really learn from your mistakes, because it takes at least a few games before you get someone's playstyle down. That's another issue with this game is the amount of MUs there are, it's very difficult to learn 60+ MUs and the different ways people spam in crappy connections.
At a certain point I was raging pretty hard at the game believe me playing Shiek on Wifi is insufferable, and the character herself is extremely difficult to master and has annoying weaknesses, imagine losing to Doc Mario when he's at 180% and you're at 35% only to get F-Smashed when you get hit once to to the ledge and now you're dead. This game gets pretty ridiculous and insane, and Nintendo has done a great job at making this game extremely accessible to younger audiences. They want your 10 year old cousin to stand a better chance playing you than he did before. That's why they made so many moves so much safer than they used to be. Kirby Up B spam is good in this game, all the Link's D-Air is good, Yoshi's can spam Down B and it's pretty safe because of super armor and reduced landing lag. All the dumb moves that us vets are used to thinking are unsafe are much safer than they used to be and it screws us hard.
And to make matters worse rage is a pretty big factor in this game, ie when someone's losing they have a better chance at that comeback something that happens to me all the time because Shiek sucks at killing. I actually think rage is a worse mechanic than tripping it's so frustrating to play against a built in comeback factor again made for your little cousin to stand a chance in the game. I know people are going to hate what I'm about to say but please understand I've been playing Smash competitively since Melee, I have 8 characters in Elite Smash (Bowser Jr. Peach, Daisy, Lucina, Pikachu, Inkling Girl, Metaknight, Young Link) so I have a decent understanding of Smash and competitive play at the very least.
This game is not competitive, or at least it's not a good competitive game. So stop getting worked up about it, you can get better, but in the end of the day this game while not shallow can be played with very little skill and you'll still be pretty good at it. Especially online but even offline these issues still exist because so much stuff is much safer in this game, grabs have been severely nerfed in this game, shields were severely nerfed, all of this effects punish game. Making spamming even better because the punish game in this game is so abysmal. Honestly I stopped caring that's what got me to stop raging and giving a crap, I play the game for fun when I'm seeing too much dumb stuff online I just turn off the game and forget about it. No more yelling about how dumb this game is or caring about this game it's just a game but not a very good competitive one. Once you realize this game kinda sucks you play it less to be better at it and more just to kill time or have fun with it because most people I doubt are even having fun online when they play this game. I'll explain in the next paragraph...
Most people who I play online will never rematch me win or lose, but especially when they win obviously. I notice it's typically actually only good players who will rematch, the gimmick ones never do. Most people will spam projectiles and unsafe moves, dashing back F-Smash is a read sure, but when they're doing it 5 times in a row it's just dumb stuff, these are the people who think they're good because they dashed back F-Smash three times in a row and you landed on it trying to whiff punish them with an aerial. They're trash players and we all know that, if they were seen on a stream everyone would realize how stupid they look but it's gonna work because this game is bad and the online is even worse. Most people will rage at this game and go try hard mode, I notice people becoming super aggro when I have the lead trying their best to hit me, I realize I'm not the only one raging at this game, half the people I'm playing are raging at this game at least, I can see it in their playstyle. This made me realize how stupid this game makes people act and was even more self reflection for me that this game is stupid and not worth getting mad about, think about how many people are acting like fools over this game, do you want to be one of them? Over this trash? No thanks. Lastly, people will camp you super hard even when they're losing by huge margins (ie they're 100% on their last stock and I'm mid 50% on my second stock) this has actually worked to make a comeback against me because when someone's camping so hard when you think they're giving up you run into dumb stuff. I don't know how insufferable you can be to camp someone when you're about to lose by a wide margin in a laggy online quick play match but I can't believe you are having fun with this game if you are doing that. There is literally nothing fun about the game when you make the match this way.
So yeah just stop caring about this game, it's dumb....realize that and just have fun with it like it's actually a party game. This is not a competitive game even if everyone wants to pretend it is, this is not Melee in the slightest or any kind of fighter that requires more skill than basic inputs. Online will only exaggerate the faults of this game to a much higher degree maybe some of these players are technical and they're only playing this way because this game promotes this type of play style. Who knows, and who cares, even though many players including myself are trying to make this game into something more it really is just too accessible to a fault.