It's times like those where I really question why I play Smash Ultimate online. You could just try to play through it, but I just find that when I get disheartened like that I don't play at my best; I make dumb plays, I react more slowly, and I don't try to adapt to my opponent. So, when it gets to that point, I don't learn anything and each defeat just feeds into the next.
There's a couple things I do. First, if I'm really not having any success, I'll switch characters. First, I'll try my secondaries, then, if that doesn't work, I'll move on to characters I'd like to learn with, but haven't used all that much (a tertiary for lack of a better word). Then, I might just try a random character that I have no expectations with. However, to be honest, if I truly am disheartened it is sometimes better just to move on to a different game for a bit and come back to Smash when I get the itch to play again. That might be a day or even a week, but I find it easier to pick up the game again when I can come back refreshed.
It's also important to take some time in training mode to practice. Things won't improve for you unless you can master skills you currently lack or combos you can't do. Let's face it, if someone is dominating you, they've likely put in the time.
As for 4-player FFAs, that can be a crapshoot at the best of times, so I wouldn't get too disheartened for losing. That's a prime situation where I'd just set my character to random and try to play casually. However, I'm not sure you can reasonably expect someone who is winning to give up their spot. Sure, it might be honourable to give up your spot if you're constantly winning, but there's no rule, unwritten or otherwise, that requires players to do that. Now, if they are engaging in unsportsmanlike behaviour or are really laggy, I might kick them just to keep my arena active, I would never kick just because someone is winning too much.
Actually what really frustrates me in those FFAs where someone is winning constantly is not them, it's everyone else. For some reason the other players never seem to think "hey, that guy is winning all the time, so maybe if I work with my other opponents, I'll have a better shot at winning." Instead they just lash out at anything nearby without any thought of strategy.
