Well, OJ Mayo played the sixth man role in Memphis and didn't really thrive. You can say he was unhappy and unmotivated there, but I suppose it's because he's a volume scorer who needs to shoot.
It's just a sample of size to make a comparison. Lee is definitely a superior defender. Actually, compared to Mayo, Nate Robinson may be a superior defender, too.
Mayo has been shooting like crazy so far, but as I said, it's just a couple of games. For comparison, the favourite for the most developed player prize, Klay Thompson, has shot 33% from the field so far.
Yet, I'd definitely not trade Klay for Mayo, if I was GS.
Of course, we don't have someone like Klay. We have Lee.
But when I look at it from the team's perspective, Lee fits in better. There was definitely not going to be enough minutes for Mayo, anyway, with Bradley back in roster and Terry playing the 6th man role.
The only way this was happening was to get Mayo instead of Terry and I'd be reluctant to make such a choice.