It is evident from other games (like smash melee) that studying and reviewing your own replays is one of the best ways to improve. In that game (melee), it's much easier to see what's going on and what different plays might have entailed. For Splatoon, it's much more complex.
What in particular should be focused on when reviewing a replay beyond how you might approach a single interaction better?
zyf_ already made a good explanation in that you'd want to identify situations that your team wiped, there was a bad death or otherwise mistakes were made - but to add to the explanation, how can you make them better?
When you've identified portions that mistakes were made on or a potential loss was caused due to, you want to consider the options that could've been done differently. A lot of the time when I've reviewed games or player skill, it tends to be aim or movement rather frequently, which tend to be sloppy. But if neither of those aren't as problematic, then you have to extend that search to different aspects: could you have used a different route, should you have waited to use your special later or you used it too early, could you have backed away earlier or waited longer, was a certain individual called out for the team or not, did the team coordinate well, so on and so forth.
Information is your key component to finding out what could be improved, not only should you expand how much you know, but how you process the information that you've gathered. It helps in the long run when you ask questions, no matter how mundane or simple, about whatever you do or have done, improving efficiency as a whole.