The California* Angels have one of the better catchers around on their team. And by that, I mean a guy who's 7th in the majors in fWAR over the past three seasons. You know, Mike Napoli. Given that, it seems a little curious why Jeff Mathis would get the majority of the playing time behind the dish over that time (even if only by 11 games or so) given that he can't hit at all. At. All.
* The joke is way overplayed, but I honestly typed it sincerely. Sorry LAAA. Unrelated, but I like the above picture because it looks like Mathis is swinging and missing the pitch by two feet (actually hitting a sac fly).
From 2008 to 2010, while Napoli was knocking out a .843 OPS, Mathis was "hitting" .200/.264/.303. For players with at least 800 plate appearances over that time, Mathis' .252 wOBA is the worst in the majors by 20 points. He actually seems to have not completely terrible plate discipline - walking at a 7.1% clip (though just 2.8% in 2010) - but the combination of not making enough contact (his 30.6% strike-out rate was 11th worst, right between Jim Thome and Ryan Howard) and not doing all that much with the ball when he does (a .259 BABIP, though with a surprisingly decent .103 ISO) makes him a huge liability in the line-up. Best case scenario, you're probably looking at a guy who's costing you about 12-13 runs with the bat relative to average (250 plate appearances) .
Now, the offensive bar at catcher isn't high, but it's high enough that Mathis isn't getting on the "productive player" ride without a lot of positive value coming from his defense. Though he supposedly has a very good reputation behind the plate*, the stats don't seem to back it up. When Matt Klaassen did his catcher defense rankings for 2010, Mathis came out dead last at -9.4 runs. Mathis didn't do a good job at throwing out opposing base-stealers (-2.8 runs, only 20% catch rate), and he wasn't great at that whole "stopping the ball" thing either (-5.6 runs on wild pitches and passed ball. Defensive Runs Saved looked more favorably upon him, coming out to +1 for 2010 and +7 over the last three years. Total Zone has Mathis somewhere in between at -6 runs last year and -3 for 2008-10. The Fan Scouting Report is similar (-3 in '10, -4 in '09 and '10 combined).
* Nichols' Law?
Game calling and whatnot isn't something that can really be measured well currently, I think, but comparing Mathis and Napoli by pitcher (just for a few starters they both caught a fair amount) might not be 100% useless:
Mathis looks like perhaps he's somewhat better with pitchers. Maybe. Not exactly a slam-dunk win though, and Napoli certainly doesn't have the rep of being a good catcher.
So overall, I think calling Mathis average with the glove is more than fair. FanGraphs has used figures between -1 and +1 for his fielding runs each season, so major adjustments to their WAR numbers aren't needed. For the record, Mathis' fWAR the last three years; -0.6, +0.1, -0.1. That keeps him out of worst player in the majors territory (Tony Pena leads the way for 2008-10 at -2.7 fWAR, and Mathis is outside the bottom 30), but a replacement level player with little upside doesn't seem like the kind of guy a contender should choose to run out there too often. And frankly, even if Mathis is a +10 catcher in a full season, the Angels are still maxing out at around 1 win over a full season.