Yea I don't think she is a good counter, her rockets are loud and slow.
And bastion worth half his salt would move away before she even has a chance to kill him, or he'd just kill her since she is so squishy.
Getting bastion to move basically works well enough for a team to do anything. Pharah can peak shot him well, though she is dead if she goes airborne.
I think she's a better counter than the ever popular genji which literally only works if the bastion nonstop fires right into an obvious deflect.