In the banks I've been to, the tellers are for quick in-and-out tasks like depositing or withdrawing money, and I'd no more expect to sit down for that than I would expect to sit down when ordering a cup of coffee or purchasing groceries. For longer tasks, like opening/closing/adjusting accounts, they have bankers with desks and chairs for all.

That said, I certainly would have no problem if the teller was sitting, even though I wouldn't expect to sit myself.