Looking at this thing with the emotions of a fan, but trying to use logic (what a concept, eh?), I feel like the Sox can be qualified as being both competitive and mediocre.
I refer to cwsox's posts as my basis for saying they are competitive. They have a legitimate chance of making the playoffs every season.
Again referring to cwsox's post, he lists several teams that have extended runs of poor records. The Sox are certainly not in that group. Then you have the teams that have had, generally, success over the years. Seattle, Oakland, Yanks, Boston, Atlanta, Giants, D-Backs and the Fish ( the last two have won WS, so that, to me, qualifies them). The Sox are not in this group either. We are basically in the middle of the pack, outside the context of the AL Central and looking at MLB overall. During the JM regime, we just a few games over .500 and that defines mediocrity.