Some matches are pure RNG if you win or lose due to the nature of there being no role restrictions, you are also stuck as the class you queued as, ontop of how fast games can swing back and forth. I've had a game where we had 3 Tanks and 2 DPS and somehow won against a more balanced composition. I don't think the game mode has had too much thought put into it in the grand scheme of things past being a quick casual muck about, so it works well as a casual PvP experience but I don't think it should have a Ranked gamemode in it's current state until they've balanced it out properly and got more player feedback first to adjust the systems and matchmaking to account for any shortcomings the gamemode may have, so it can provide a better experience for competitive play (at least for ranked).

I've reached Plat 4, almost hit Plat 3 before going back and forth on stars struggling to climb up.

Generally most of the time, the team which wins is the one that focuses on which target to burst and doesn't waste all their oGCDs on a target that is guarded and try to snowball whilst maintaining kills so the enemy team is unable to push them off the objective whilst gaining as much ground as possible.

I think right now, there is not really any measure of "getting good" since there's too many variables within CC that are out of your control as a solo player due to how it currently is, I mean after you've gotten to grips with how your stuff works, that's kind of it, in MMO PvP, the "getting good" part would be the teamplay aspects and the problem is, that's hardly a thing right now since roles are unlocked and the system is just matching whoever with whatever together, so it's a complete and utter chaotic mess just like how Frontlines can be but at least in Frontlines however your communication isn't limited to just callouts and games last a bit longer so there is some strategy involved to some degree! CC is just far too fast-paced in my opinion.

As for which classes are pretty strong right now, I'd say it'd be Warrior, Dragoon, Red Mage, Summoner, Samurai and Ninja.

Warrior has an AoE engage stun and lots of AoE in general, it's LB also disables Guard so if you have a damage heavy team, that basically wins fights if your team is prepped to take advantage of it.
Dragoon currently does the most melee damage in burst if left unattended, it's LB is also very strong for both killing and warding people away from the objective depending on how you use it.
I've seen a lot of successful RDMs and SMNs, though I haven't played them myself.
Samurai has a very strong kit for both 1v1s and teamfights. It's LB can insta-kill people.
Ninja can chain executions with it's LB and is generally a big nuisance if left unattended and is the only melee DPS class with a ranged attack currently.

Though all of this information is truly subjective and until they make adjustments to both the systems and the classes, take it all with a grain of salt.