I suppose it helps if a critic knows something about music, but I would argue knowledge of the art is secondary to the critic's ability to articulate what it is about a performance or a piece of music that would recommend or dissuade listeners, not to other musicians, who, one would hope, can decide such matters independently.

Can the critic translate the performance or the new magnum opus into terms that concert goers can understand? If they are able to do that, they render a public service approximately equal to providing  a traffic report that will put drivers on the most expeditious route.