Mediocre would be 6th-8th man on a middling team. Although the Kings didn't really make it to the "middling" level last year, NJ was as middling as possible (41-41), so if you take Moore and Salmons together, they might squeak by at the low end of mediocre. But when teams like GS and the Clippers have Brandan Wright and Al Thornton as 6th-7th men, it really is just squeaking by.
I don't think that "sub-mediocre" is much more of an exaggeration than "mediocre" would be. And when Moore turns 33, "sub-mediocre" will probably sound exactly right.
If Mediocre is to Par
Then sub-mediocre is to Double Boogie
There's a big difference between average and below average.