If you say you're a pianist, and others ask you to play, and turns out you can't play a single song properly -- chances are, some of them will now look at you as a bit of a poor judgement fool.
If one is fine with that, then yes, call oneself a pianist I guess?
Personally I'd say I was "learning to play the piano".
Or what about someone says: "I'm a Software Engineer" ... but then can't write the first characters in a FizzBuzz solution and wonders "what's a for loop?"