The only place where level 200+ significantly affects matchmaking is Arena, so long as we make room for some pathological cases.
In Arena, that’s the point where they stop filtering the high-level weapons from play. There are two ways to view this.
The cynic: The most famous of these is Dawnbringer, a tool designed to destroy weak teams in one fell swoop. This makes it ideal for endgamers to use Arena as trophy farming. It’s very difficult for low-level or inexperienced players to consistently beat it. Since it costs 1.3 million souls to construct, you’ll have forgotten about Arena by the time you get it.
The endgamer: this is a good challenge, I beat Dawnbringer all the time. Just get better at the game, make good choices, and use your mythic troops. Arena’s the best mode. If you’re losing, you should go play Connect Four or something more your speed.
In PvP, there are many more factors. Your level DOES affect matchmaking, but the system doesn’t throw you at endgame parties right out of the gate. You’ll face other teams near level 200, but now you’ll also have a chance to face the people who, by luck or by money, have managed to start assembling high-end teams very early. But more often than not, you’ll face teams you have a pretty good chance of beating.
I found that before level 900 or so, the thing that most dramatically affected my PvP matchmaking was “my team”. Upgrading to traited legendaries/mythics usually resulted in a very clear spike in difficulty. Sometimes it took a month or more for it to kind of smooth out. This makes teambuilding a balance between “I want to win faster/more” and “I’m afraid I’ll hurt my win rate if I advance too much”.
But, eventually, your level will push higher-end opponents your way. I don’t know if it’s worth worrying about it. I definitely had some low points, but I think if they’re enough to make you quit, I can’t be assed to complain about, “Oh no, you found a different game that was more fun for you.”