I'm at 50 and my avatars are at 70. I'm getting zero on scores. What do I do?

Indeed, once you reach level 50 and have upgraded your main 4 classes, it gets harder to earn credits in the game.

However, you will earn a certain amount of credits per match you play, so you still earn credits, just way less. If you want more credits you could for example level up other classes such as the enforcer/jumptrooper etc. 

Many in the community have already asked for them to higher the level cap from 50 to 100 or something, so players can work towards something again.

Also, if you want to buy skins, you could also buy crystals and buy them with real money. But that is your choice!