Old Man Nintendo
Your post seems tacit -- I feel like I have to read into the header of your post more than than the body's contents in order to understand what you are getting at. "I'm Absolute Trash Online" is an interesting draw, but the message that follows meanders away from the initial thought. I'm stuck having to work off presumptions based on my understanding of the intent behind the message, and so that is what I will do until corrected.
Primarily, the intent seems to be seeking advice in understanding - for lack of a better term - the
online meta. I presume that most of your skills in Smash come from offline play and probably from a different Smash game, if not a completely different fighting game altogether. I would assume the former over the latter. If this is the case, then your main prerogative would be to understand the main elements of online play, contrast them to what you know of offline play, and then modify your current choices and techniques in order to increase your chances at victory.
The advice that yknolikenia gives in regards to characters playing differently within the online arena is something to take into consideration. For someone who seems more or less unfamiliar with online as opposed to local play, I'd actually say not to take into account input delay and such. Instead, play a variety of different characters to get a feel for what works best for you. Mostly it's to find a fighter(s) that you are comfortable using online, but it also serves as a way familiarize yourself with online play while focusing on something other than latency. The main thing to learn here is that the more you play online the better you get at playing online. The more you learn a character online, the better you understand the intricacies of online play itself.
One important thing to know, however, is that the Nintendo Switch Online has notoriously poor service. Most people point to netcode (how the game inherently works with online connections), and
many words have been written on that subject -- I will let you research for yourself if you ever find the desire to do so. This means you shouldn't ever get
too hung up over how you play, playing well
or playing poorly, because both could be products of netcode/latency over genuine skill. Because of latency, online play is
never a fair game. The game will interpret how it
thinks the game is being played and then will adjust things accordingly. This could take effect as connection 'stuttering', dropped inputs, misinterpreted inputs, teleportation, speed-up/slow-down timing, or a connection that gets dropped altogether.
I'd honestly suggest getting a wired connection because, in a perfect world, we'd all have wired connections. But the truth is that a great number of people out there are unwilling or unable to get a wired connection (read: johns) and some won't even make sure to at least play with a strong wireless connection (full signal). This means that there's a good chance that any given game will come with connection issues and even the games that run smoothly will still have latency problems. Just keep in mind that if you are enjoying a "lag free game", chances are that it's because you're currently hosting and that means your opponent is probably shouldering the ping distance. Its a good exercise in humility to acknowledge that the opponent is playing their current best at all times rather than judging someone as "bad" (because they're likely just having a bad internet day).
Most of what I've said has actually come from years of playing Splatoon 2 (which is basically only played online multi-player), not SSBU, but I'm pretty confident that it applies equally well. Sadly most of my advice just boils down to "play more" and "get a wired connection", but that's effectively how you play online... although... if you are finding great difficulty in your online success you could maybe consider troubleshooting your network connection. I know it's generally a taboo thing to say this among the WiFi warriors, but the truth is that some people just have better internet than others.
