A mellow beverage is one that is "matured" and isn't "raw." A mellow beverage has a sedate flavor, one that isn't jarring or over-the-top. This can be a good trait or a bad trait, depending on the style of beverage and your preferences. For example, a "mellow sauvignon blanc" probably wouldn't be a good thing, but a "mellow vodka" certainly could be. (Wine,Beer,Spirits,Sake,Mead/Tasting Terms)