Counter-teamcomp in bronze and silver is important, you can't just carry but learn to play a handful of characters semi-decently.
Also cancer comps are stronger on this rank where everyone has shit aim/positioning/game sense so learn to play characters that will counter cancer low effort/high reward heroes like mercy/symmetra/torbjorn/junkrat/bastion.
D.va is good for KOTH, I pretty much can carry myself on KOTH fights wirh her, but I recommend you to learn winston (for symmetra/mercy), pharah (for spamrats, also nobody can aim so it's effective), junkrat (for bastion/torbjorn also people with shit positioning will run to your spam willingly) and mercy should be your go to healer, you can undo your bad team mates' mistakes/compensate them.
Tips: -soldier 76/reaper's ult can wip an entire team of bronzes during teamfights because of their positioning.
-Never underrate an enemy symmetra/bastion/torbjorn and always switch to their counters, otherwise they gonna stall tf out of your bad teammates.
-Always check the enemy team's most used heroes and try to find a common counter, sometimes you'll run into a mercy and a genji main, go winston.
-Have fun!