Solved

-10 repo is ok Inspite wining?

After winning nearly each race -10 repo is justified.
I understand repo of league might be low. But by the end of season m loosing 70 repo in all and my presence not helping to improve overall repo of league.
Let me know how I can improve it.

I think that's rep working as intended since rep of other managers are quite low.

2 options either get high rep players to join your league or you leave and join another league! The way rep works is if you don' have tough competition even though you win you lose rep.