When it comes to gameplay, the most important thing about Mazzy is that she is a Fighter. Because of that, she's not much different from Korgan. I look at it this way. Mazzy's stats don't make much difference because I can use items to overcome any of her shortcomings. If she's wearing a Girdle of Hill Giant Strength, it doesn't matter what her natural Strength is. (By the same token, Keldorn's natural Dexterity doesn't matter since he can wear the Gauntlets of Dexterity.) Mazzy's natural Dexterity of 18 gives her a better armor class than Korgan (unless Korgan is wearing the Gauntlets of Dexterity), which helps make up for the fact that she has fewer hit points than Korgan. At most, he has about 18 more hit points than she does, but his armor class is 3 points worse than hers, so she gets hit less often than he does.
In combat, the most important things are THAC0, attacks per round, damage output, special attacks, armor class, hit points, special resistances, and saving throws. Here's a breakdown.
THAC0: All warriors have the same THAC0 if they're the same level and have the same Strength bonus and item bonuses. A girdle of giant strength and a +4 weapon give all warriors the same THAC0.
Attacks per round: This is determined by level and proficiency points. Mazzy can put five proficiency points in any weapon type, so she can have the maximum number of attacks with any weapon. Keldorn can only have two proficiency points maximum, and Korgan can only have one or two points in ranged weapon types, so they are somewhat more limited.
Damage output: This is determined by the weapon being used, Strength modifiers, and special items such as the Gauntlets of Weapon Expertise. Cavaliers and Rangers get special bonuses against certain kinds of monsters, but Inquisitors, Fighters, and Berserkers do not. Mazzy can wield almost any weapon in the game (there are a few that are restricted by alignment or class), and with a girdle of giant strength, she can hit as hard as anyone else. If you give her proficiency points in Warhammer and give her Crom Faeyr, then...well, anyone who wields Crom Faeyr can be a tank.
Special attacks: Fighters don't usually have any special attacks unless they use special items. Keldorn has the ability to cast Dispel Magic, which is one of the most powerful abilities in the game. Korgan can go Berserk a few times a day. In my opinion, this is the only place where Mazzy is at a disadvantage compared to Korgan. In Throne of Bhaal, Mazzy can get High Level Abilities just like other Fighters.
Armor class: This is determined by armor (and shield) and the Dexterity modifier. Mazzy can wear almost any armor in the game (some are restricted by alignment, and she can't wear Keldorn's armor, for example) and she can use any shield. That combined with her high Dexterity makes her a great tank because she's hard to hit.
Hit points: Mazzy has the hit dice of a Fighter and Constitution 16, so she has more hit points than most characters. Keldorn has CON 17 and Korgan has CON 19, so they have more hit points than Mazzy, but if you don't let Mazzy get below 10 hit points (my characters' hit points hardly ever get that low) then you probably won't notice the difference.
Special resistances: Mazzy can cast Invoke Courage, which I think has the same effect as Resist Fear and Remove Fear, which comes in handy sometimes. She can use most items in the game, so you can give her the Ring of Free Action and/or Boots of Grounding, for example. Korgan can use Berserker Rage a few times a day.
Saving throws: Mazzy has the saving throws of a Fighter just like Korgan, which are pretty good. Keldorn has the saving throws of a Paladin, so his are better than Mazzy's and Korgan's. Mazzy's saving throws can be improved by using various items.
If Mazzy was a Cleric or she couldn't use items like the Girdle of Hill Giant Strength, then you might have doubts about whether she could be your main tank. But if you think of her as a Fighter and look at ways you can maximize her abilities, I think you'll see there's no reason to doubt her. In fact, she can really kick some butt.
Try the following equipment and you can send her up against any opponent:
Sword of Arvoreen (until you find something better or change her proficiency points)
Armor of the Hart +3
Shield of Harmony +2 (or Dragon Shield +2)
Helm of Balduran
Bracers of Blinding Strike
Amulet of Magic Resistance
Cloak of Protection
Girdle of Hill Giant Strength
Boots of Grounding
Ring of Free Action
Ring of Regeneration
Best of all, unlike every other NPC in the game, Mazzy NEVER whines or grumbles about ANYTHING. She has a heart of gold and nerves of steel, and she's the bravest warrior you'll ever meet.