There’s definitely a grey area when it comes to what’s good but also there’s usually a pretty noticeable area of moves that are objectively more good than bad. Response above pretty much said it all
Apart from that it's moreso about patience & predictability. The longer you play usually that starts to fall off...so try to have a couple gameplans when certain moves arent working, then cycle back to moves you might have stopped using. The more niche ones fit that role because while people look for them out of habit, sticking them in once a game or 3 can be very effective. Sometimes just the threat, even if whiffed can be good enough to make people move awkwardly. Think moves like tremolo...very niche, narrow window of use...but it happens once or twice.
Certain moves are good for punishing, others coming out basically when you press the button effectively speaking, then the slower moves you really gotta time/practice with for reads or when opponent moves suddenly or suddenly stays still. Those moves catching people in transition (standing still or about to move) are some of the hardest to get down because it requires you to recognize where the opponent likes to congregate on screen & their movement patterns. Easier said than done, but think of your moves like a kit in an rpg. You have moves moreso for defense (when youre backing up/standing your ground) then offense (pushing to corner/moving forward). You'll notice overlap and some are almost exclusive to one of the two. For characters like squigly, she has more defensive/transition-catching moves for specials & divekick is her main offensive skill. Seria cancelling overlapping both offensive/defensive use. (Usually) The kit with the most moves is where you wanna lean to get conversions to get on offense.
Sidenote, when people get 3/4 of the stage away from you, usually a good time to employ those awkward squigly moves because many characters cant do too much to immediately effect you from there & have no where to go but forward quite literally...so throwing out chord or tremolo is a good bet. obviously watch out for supers & how much meter they have
Find your favorite moves then think about every instance or way you can use them. Once you're able to decide which moves you want to use for what, your time will be far easier & from there you can build on that