What makes you think your lack of tip doesn't impact your rating now? The few Uber drivers I know expect a tip.
I think this varies a lot by city. I don't know if you're in San Francisco, but it's never been my experience here that drivers expect a tip. But when I fly down to Las Vegas nearly all the Ubers I've taken have notes taped up in the back stating the driver expects to be tipped (in cash).
It would annoy me to no end, and I would be surprised if Uber would have been cool with that before now.