If your data usage comes from inefficiencies of Pokémon Go, then it is Niantic's responsibility. (At least up to however much bandwidth they could have saved.)
No, but we do expect that it's reasonably optimised. For instance, no one expect a reasonable app to download the same visual assets over and over again (and it looks like Pokemon Go does just that, though I'm not entirely sure I believe it).
It also runs against Silicon Valley culture, which is to get to MVP as quickly as possible and release early. There are trade-offs to be made between complete optimization and time-to-market. Leaning entirely in favor of the former is almost never the correct decision, and certainly not in startup land (and Niantic is a startup!).
