(context for anyone who only knows me as a game developer: i was a competitive fighting game player for a long time and i won evo in 2021)
that's a great question!
the most general answer i can give is that getting better at fighting games is just a matter of playing matches, analyzing those matches to see how you could improve, putting in the time to commit those improvements to muscle memory, then repeating the process. it's okay if you play more matches against those same opponents and lose again, as long as you don't lose the same way that you lost the last time. high level players are just players who have been through that process more times than the mid level players and who have patched more weaknesses in their skillset and gameplan. but that's a dull answer so i'll attempt to be more specific.
in my experience as a skullgirls player, mid-level players usually tended to have at least one huge gaping weakness in their skillset. it was much more common to see a mid-level player who was almost as good as high level players at a few things and terrible at some other things than it was to see a mid-level player who was average at everything.
back in early 2018 i wrote some articles on how to overcome the biggest mistakes that i kept seeing mid-level skullgirls players making. despite usually being strong at running unpredictable mixups and converting their hits into strong combos and resets, those players usually struggled to...
adapt in neutral. for example, they would just run the same neutral strategy against everyone instead of tracking and taking advantage of their opponents' habits in neutral, or they would have a few predictable bottlenecks in their neutral game that high level players could take advantage of.
get the most out of each hit. for example, they would convert every clean hit into their optimal damage combo or strongest reset setups instead of thinking about the nuances of the matchup against the opposing team and how they could best play into that matchup by saving, spending or denying resources as appropriate, or how they could track and take advantage of their opponent's mental state and catching them with suboptimal resets when they're not expecting one.
make comebacks. for example, they would mentally give up when they were down to a sliver of health, get hit by very predictable chip setups from their opponents because they weren't confident enough to make a hard read, or wouldn't have learned the most efficient way to take out an entire opposing team with their anchor character in as few clean hits as possible.
in short, mid-level skullgirls players usually had a solid layer 1 gameplan with good mixups and high damage, but they often struggled to change their gameplan (both in neutral and after landing a hit) based on the situation/matchup/opponent.
i imagine those specific weaknesses will be different depending on the game. it's so much harder to consistently make good choices about what to do after you land a clean hit in skullgirls with its teams and assists and resets and meter scaling (both on the attacker and the defender) and persistent undizzy and extreme matchup variety than it is in a game where you just do your standard combo and run your oki, but it's relatively easy to snowball a single lucky hit into a win against another mid-level player in skullgirls compared to a game that regularly resets your opponent to neutral. but i imagine the general pattern will be the same: mid-level players are often great at a few highly rewarding aspects of the game (e.g. running their layer 1 gameplan and blocking their opponents' mixups/oki) but very weak at other things. it's just a matter of learning that those other skills even exist then putting in the effort to practice them.
