Is the output of a Mealy/Moore automaton a regular language?
By discarding the input-letters you only consider the output language of the respective Mealy automaton. Effectively you obtain a finite state automaton that recognizes the output language and because it is a finite state automaton, which may be non-deterministic, the language it accepts is regular.

The length of the output of a Mealy-transition is irrelevant as you may have for example$aa\in\Gamma$, where $\Gamma$ denotes the output alphabet of a Mealy automaton.