Medium-sized, with 10 outsider RHD and actual item-holding limbs, as well as a horrible outdated description (Teleport Without Error hasn't been a thing for a while now, guys).
In terms of stats, they are... underwhelming. 16 strength isn't just below average for a monster, it's something an orc could innately have by now, considering that ability increases are a thing. 12 dexterity is meh, 21 constitution is, uniquely for this thing's stats, acceptable, and the less said about 7/10/14 mental stats the better. I guess those abilities are kind-of on par with things like the bearded devil (which has a similar role and CR), but at least the bearded devil isn't weighed down by this many HD.
In terms of special abilities, mezzoloths are hardly special either. Telepathy, DR/good, acid and poison immunity, SR, energy resistances... We've all seen it before. The SLAs are underwhelming (at-will Cause Fear, Darkness, Desecrate, Produce Flame and See Invisibility are terrible, 2/day Cloudkill, Greater Teleport, and Dispel Magic are okay I guess). The Summon Yugoloth ability is somehow worse than the canoloth's, just to add insult to injury.
Anyway: is the mezzoloth worth +0 LA? I'm not sure it is. It's got melee abilities on par with an orc warrior, and the two cloudkills it can drop each day don't change that much. It's got defenses that, while okay, aren't impressive, and no real ability to draw aggro. It doesn't have class features, or anything worth sinking 10 levels in. The same ten levels, naturally, also make it so that by the time they can enter Fiend of Possession, a character of equal level could be done with it already.
-0 for now, on account of massive RHD bloat. Maybe remove half of it and see if the mezzoloth improves.
(Also, speaking of Cloudkill: why was it put on a CR 6 monster? Don't get me wrong, it's nearly useless by mid-levels, but at ECL 6 the PCs will die if they fail a save, and they won't have the magic to come back. Subjecting the entire party to a SoD, twice, seems just a little rough for what should be a fairly typical CR 6 encounter. And that's just one-on-one battles, don't get me started on what's going to happen if a CR 5 canoloth gets lucky when summoning one.)