The proxy server doesn’t need to be an actual Nostr relay, and could have even a different API, devised to be optimal for mobile apps.
However, that might make adoption harder.
I know there’s some work going on about proxy relays, but I’m not sure it’s the same thing.
Also note that you need to trust the proxy, or run a proxy yourself (and only have to trust that the set of relays you’re connecting to isn’t censoring or manipulating your feed).