Serviceable and what does it mean.... That's an interesting point. As fans we tend to compare one player against another. Is Hanley a better 1B than Moreland? Is Nunez better than Holt at any position? Is Devers 'serviceable' at 3B?
Long story short, "serviceable" to me means that he's not hurting the team, which is really part of the criteria for a player. Anything beyond that is a plus.
When I used that word to describe JDM in the outfield I didn't mean it as a knock on JDM. What I meant was that IMO (and without seeing every outfielder in baseball) it was my opinion that he's at least average now defensively. However, my opinion may be skewed because, as I said, I don't see every outfielder in baseball so I tend to compare JDM against the two I see the most of - Mookie and JBJ.
I don't know what the defensive metrics say about JDM as an outfielder and I don't much care. The more discussion I hear about defensive metrics the less faith I have in them especially as they pertain to outfielders. All I care about with JDM in the outfield is that he goes out there and gets at least an average job done - which he's doing now without the drama or misjudged balls.