from what I have seen, I am pretty certain it is solely dependend on your tower rank vs opponents tower rank how much mojo you win/lose. If you fight someone that would give you 32 mojo you will win 32 and lose maybe 0 or 1, if you fight someone that gives you 1 mojo, you will lose 32.
so there are 2 strategies to follow - you either hope that you above average ego for your rank and take every fight, which will give you an overall gain, or you just do not take fights against obviously stronger opponents (which I personally do not like as 2 crits in a row can win or lose you a match, it happens frequently that you get clapped by someone with 20k less mojo and then you suddenly win against someone with much higher mojo)