He doesn't approve, but he also doesn't mind what we do (so far) as long as he continues to survive and find pleasure in life.
His pleasure in life comes often at the expense of others, though. If you play the good path and won't let Astarion drink from Tav (which seems to be a large approval boost) & don't allow him to drink from other intelligent beings, you can end up with low approval. I never got him to leave the party, though maybe it is because the writers are setting him up for betrayal? On the other hand letting Astarion drink blood, while making him happy, feels like a "trap" choice for a good aligned character. It allows him to engage in his monstrous nature for the first time, something he couldn't before, ironically due to Cazador's orders. So it is a step away from redemption.
