It happens all the time.

You reach Rank 1 - 2 and then you have to battle opponents with 3+ mythical items and you got none...
And this multiple times
Bye bye Rank 1 ... hello 2. Or worse.

Why not make the matches a little bit more fair?
You could give the inferior mech a little boost like in campaign fighting insane mechs depending on the difference of mythicals.

The inferior mech can choose between 3 bonuses. One per mythical the superior mech got more.

Example1:
Player 1 got 1 mythical.
Player 2 got 2.

Player one can now choose 1 bonus out of three.

Example2:
Player 1 got 1 mythical.
Player 2 got 3.

Player one can now choose 2 bonus out of three.

Is the difference three or more, the inferior gets all three bonuses.

Well, For starters, Your Rank 1. Thats the highest, But i do think they should have league after rank 1.
Anyways, They are rank 1 aswell. They just happen to have Mythical items for being really good so they are in top clans, Or They buy Item boxes. Supermechs, Goes by Rank not Skill. But honestly, It should go by people around your own W/L Ratio. And i think your idea isnt the best. I think its Good how they stick to ressistence and Cooldowns/Generation.