It is entirely idiomatic to refer to a "nice answer", or indeed a "nice" anything, to mean 'good according to the relevant criteria'. So a nice answer is clear and helpful, a nice cake is tasty, a nice town has low crime and pleasant buildings, a nice dress has attractive fabric and a flattering cut, etc.
In fact, it is so commonplace to use "nice" as a generic positive adjective, that it is actively discouraged in some circles (e.g. creative writing courses), in favour of thinking of a more distinctive word.
On the other hand, you appear to be asking if it idiomatic to say a "nice answerer". I am not sure if this is a typo or intentional. If intentional - no, not really. I suppose it makes sense grammatically to say "I recieved a reply from a nice answerer", but it's not something I can imagine native speakers saying often, or ever.