Not a top player but from my experience, common mistakes I tend to see when I play in X rank are:
Enemy beakons not being dealt with- reason for this could be down to the player tunnel visioning and not having the spare capacity or awareness to check the map. I always assumed once splatted, you'd naturally check the map, see a squid beakon and deal with it (especially if it's near your side of the map) but I've seen players just swim right past them.
Understanding game states- There will be times where we have the man advantage but we don't advance and take the appropriate space in unison (e.g capped the zone but don't push ahead) or do the objective (e.g ride the tower/pick up the rainmaker) quickly enough. At times, you'll have a single player try to force the issue with a particular action that ends up being detrimental to the team (Pushing tower with all 4 enemies alive and in key positions or a player camping under the enemy basket in clams hoping for a counter push when the opposition in question are railing your basket in a 4v3 situation).
Squid Jumping- This feels even worse than it was in 2 and I'm not sure how it got this bad. You can probably point to some things like the state of the maps and how quickly the game is played compared to 2, but it doesn't explain why people continuously try to jump to teammates who aren't in the best position to receive jumps, get splatted easily and thus prolonging the disadvantage state.
One thing to look for other than general awareness is to keep in mind what your team and enemy team comps comprise of. It can give you a general idea of how the match can potentially play out assuming everyone in the lobby plays the weapon and their kits naturally.
