I don't think design should ever be slapped on. But I do think it makes the most sense to test and launch a first version with what amounts to a functional wireframe/prototype, if possible.
Personally, I would love to see Google in the carrier business, but it would be a huge risk to their brand/image.
Most everyone loves Google right now. Most everyone hates wireless carriers. So, it sounds like it could be a good opportunity. But there are reasons people hate carriers that are either outside of the carriers' control or too costly to fix completely on a global level (ie. spotty coverage, slow connections, great expectations, etc)
Even if Google became the #1 carrier, their valuable, mostly unblemished image could take a big hit.
Google's ability to simplify/scale technology for the masses (Google search, Gmail) won't help as much when it comes to dealing with frustrated customers on the telephone who want to speak to someone right now.