Really great programmers can quickly become passable at writing code for any platform. It does take time to learn how to do things the way a different community does them, which is why there are things like "PHP functions implemented in JavaScript" and other insane miscellanea in any language.
So what are you really asking here? If you're asking, "is it enough for me to be a mobile developer?" then the answer is YES! Many people like me love to have someone do the work we hate -- the fragmented, fickle frontend. It requires a lot more experience and knowledge than some people think.
If you're asking, "can I find someone who knows both?" then I do think you can, but you're making it harder on yourself. Someone who's a 10/10 on the backend and 10/10 on the frontend is a unicorn. You probably won't find that ever.
The two facets (frontend and backend) are different enough that it's worth hiring specialists. Get someone who's a ten in either category, put them together, and I guarantee you'll end up spending less time/money overall. I have some anecdotal support for this because my company just did this with two apps after a single person wrote one of the prototypes.