How can I level out this hanging light fixture?
To me it looks like the arms might have been bent/rotated slightly which causes the imbalance.

Try to see if the arms can be bent/rotated at the joint and try playing around with their positions.

It also looks like the chain can slide horizontally within the shackle, possibly you could balance it out a bit there as well.

Alternatively, if the metal is magnetic you could try buying some small magnets and try balancing them out like that.