Mona provides a universal DMG bonus through her burst, which also does massive damage.
The core of her kit is her q. Mona ascends with ER and has a 60 energy cost burst, similar to Bennett, Yun Jin, Venti, etc. The q will apply a bubble to enemies within a 10m radius, applying hydro. It can also lift small enemies (weight < 100) and immobilize them. When the bubble pops, it will create a huge aoe explosion and also trigger the omen effect, boosting dmg bonus by a lot. The omen lasts up to 5s (increases with talent level) but can be extended if the enemy is frozen (omen extension).
The bubble lasts 8s before automatically popping. The bubble can be popped once the enemy has taken enough poise damage (stagger). Even things like Yoimiya's E cast can pop the bubble.
A common party trick with Bennett is to cast Bennett's q, cast Mona's q, wait 2.5s (for the icd from Mona's initial hydro application to reset), then apply a pyro attack on an enemy affected by the bubble. The bubble will then explode, applying hydro again, vaporizing, and deal 2x damage.
Her e is a taunt that provides off field hydro application. The taunt does aoe damage and taunts within a 5m radius.
Mona also has an alternate sprint. Funnily enough it was disabled during the 2nd Windtrace event in Version 2.4 after it was used in the 1st Windtrace event to hide from others. Ayaka's alternate sprint was also disabled during the event.
C2 and C6 increase her ca damage, which is for if you use her as a dps. C1 increases hydro reaction damage (swirl, vaporize, EC) and extends frozen duration all by 15% on enemies affected by omen. C4 increases crit rate of party members that hit enemies affected by omen by 15%.
When dendro was released in version 3, certain effects such as the TF 4 piece effect were updated to include dendro reactions. Will Mona's C1 also be updated to increase bloom damage by 15%?
4 piece Emblem, ATK-Hydro DMG-CRIT. Noblesse, Heart of Depth also viable.
Weapons: Widsith / Skyward Atlas / Solar Pearl (build damage on her). Some hardcore Mona mains have also tried prototype amber (to give her healing abilities). Some ER catalysts are ok (but Mona usually has enough ER herself that it is not that needed). TTDS is also ok for boosting ATK, though her Q can usually do a lot of damage so building her own damage is usually better.
Because of the universal buff from her q, she can be used on basically any team for extra damage and dmg bonus, making Mona a pretty versatile unit. That added to her AoE and extra cc through her taunt, she is usually one of the best options (if not the best option) as an off field hydro support for freeze teams especially because of the omen extension through freeze.
Sometimes her buff can increase damage more than Bennett. For example, Mona can sometimes increase Itto's dps more than Bennett can. This is because Itto gets so much attack, that Bennett's buff can hit diminishing returns. However, Itto does not have as much damage bonus, so Mona's huge dmg bonus buff can increase Itto's dps a lot. For example, say Itto gets 4k attack and Bennett gives 1k more attack. Also suppose Itto has around 70% dmg bonus, and Mona gives 60% more dmg bonus. Bennett's buff increases damage by 5/4=1.25 times, whereas Mona increases damage by \(2.3/1.7 \approx 1.35\) times. So here Mona gives a bigger buff than Bennett, which illustrates how broken Mona's buff can be. (By contrast, Xiao is an example of a character who gets so much damage bonus, that Bennett's ATK buff usually gives Xiao a bigger increase in dps.)
Plus Mona can be used as a buffer on teams without Bennett. Say Bennett is used on the other team in the abyss. Then Mona can buff damage for the team in place of Bennett, making her quite flexible.
Bennett is already one of the best buffers in the game. The fact that Mona can give a bigger buff than Bennett in some situations shows how talented Mona really is.
There are only a few characters that can give a bigger buff than Bennett in certain situations: these include Mona, Yun Jin, Shenhe.
Mona's q increases dmg bonus by 60% at level 10. Among other talents that increase dmg bonus specifically, there is also Kazuha's poetics of fuubutsu (but the dmg bonus is not as much and only applies to pyro/hydro/electro/cryo). If a typical dps character has say 80% dmg bonus (some characters have way more like Xiao), Mona's buff will increase this to 140% dmg bonus which can increase overall dps by $$2.4/1.8 \approx 1.33$$ This is a good increase in damage, comparable to a TTDS ATK buff.
We assume level 10 q with 4 piece emblem, 200% ER, and a hydro dmg goblet for a total dmg bonus of $$200(.25+.2) + 46.6 + 60 = 196.6$$ factoring in the 60% bonus from Mona's omen as well as Mona's passive which increases hydro dmg by 20% of her ER. (Note there is a limit of 75% dmg bonus from the emblem set but no such limit on Mona's passive. Also a Lv90 Mona gets 32% ER from ascension and 20% ER from 2 piece Emblem, making 200% ER quite achievable.) Needless to say, Mona is a character that gets a ridiculous amount of DMG bonus innately (similar to Xiao and Raiden). If Mona vaporizes each bubble pop and spams her q on cd, her dps is $$x_b = 2*796.32*2.966/15 = 314.918016$$ Mona's burst is one of the strongest nukes in the game. Without vaporizing (say in a freeze team) $$x_b = 796.32*2.966/15 = 157.459008$$