Guild Wars 2

Which profession has the ability to do more dps in a dungeon/fractal? I read on a post that mesmer does more damage and thief is just good for mobility.

I suppose they were assuming that the Thief would be down, because with the right weapons and traits, you can do very good AoE damage (not as cheat as 100b which will be nerfed very soon) and evade everything. Lots of dodges. BLINDS.
I use S/P-P/P and barely go down in dungeon, except when I take some random huge aoe damage coming from nowhere (thanks GW2 hitbox) or not paying attention.
I can use and spam blinds which help a lot against tough enemies. You can also buff your mates with Fury+Might (Steal trait) and I can use the Charr Battle Roar which gives again the same boons. So it’s really usefull both in offense & support.

With a Thief, it actually demands a lot from the player : focus, attention to what the ennemies are doing all the time, position.
It looks hard, but it’s a very interesting way to play. And not as dumb as a Mesmer poppin’ clones/phantasms and running around in circle.
Thieves SHOULD be very good in mobility, but in reality, Eles have it better than us.

But in the end, it comes down to the class you like to play, even if I was playing a warrior doing 50.000 damage aoe with 100b I would quit, simply because playing with a class that has not the playstyle you want will be boring after a short while.
It’s not all about numbers.

It depends on the way you play it.
I think that thief can pull out higher amount of dmg easily than a mesmer but it’s more situational.
Mesmer requires phantasms to deal a realiable amount of dmg. It’s easier to play with a mesmer against multiple enemies. Definitely higher survivability (=/= from thief mobility and stealth, unless dodger thief, which would instead expose teammates to danger, tho, Shadow Refuge is among the best things you can come up with), one of the best classes for reflicting proj (very useful on fractals).
Thieves are mostly good for overall damage and offensive support by giving boons like vigor/fury and blinding enemies (with shadow refuge allowing you to res an ally easily, if not on enemy aoe).
Mesmer has more sustained survivability and defensive/ hard cc(pulls, dazes immobilize or cripple) utility which most classes don’t have. (But that doesn’t mean they do less dmg than thieves, because on occasion they can do dmg while most of the party can’t)
I for example play Phantasm S/F and Staff build, full power/crit and boon duration.
I don’t excel at dmg like a Warrior, but I help a lot my teammates against proj with focus and feedback, I’m not required to keep hitting the boss because I just need to pop phantasms to deal dmg without exposing myself in danger, and kill adds meanwhile. Chaos storm gives excellent defensive boons to the whole team, my warlocks give regen to who’s ranged and warden to who’s melee and soak up aggro and dmg allowing my team sustain dmg easily. With staff I have perma-regen, vigor and protection on, so I don’t have to worry about myself and can take more care about my teammates.

Some builds can do more dmg than other. It depends on your playstyle too.

As Far as Mesmer damage output in a dungeon, I reckon thief hits harder. To start off with, the only area where Mesmers excel in damage against a thief is shatter spike damage where you can pop a 6 clone shatter in 1.5 seconds dealing around 22k and up, similar to a thief combo. However this is situational and most of the time pvp only. In dungeons Mesmers can’t set up shatters the same, so it brings it down to 2 builds:
Phantasm
Condition.

Phantasm builds can deal good damage, when they work. Our main form of Phantasm damage is via the offhand sword which can produce high damage, focus 5 or greatsword izerker. But, izerker is bugged, and does low damage.

Condition mesmers really shine in a dungeon, when paired with a necro. The staff phantasm does 10% more damage based on each unique condition, and it is compound. So the iwarlock can hit 14k per bolt on bosses with stacks of every type of condition, but it is relativly weak without conditions.

It is basicly opinion based, and as far as mesmers go in dungeons, to do it well you have to hit your combos and fields perfectly which can be tricky, however a mesmer has higher survivability, with so many Get-me-out-of-here skills that often enough they sustain damage for longer.