Mona is a Hydro character in Genshin Impact. She uses a Catalyst to inflict Hydro damage on her enemies. Her elemental skill can act as a taunt, drawing enemy fire while dealing damage to them before exploding, dealing area of effect damage.
Her elemental burst ability will inflict Bubble status on enemies, trapping them in a pocket they cannot escape from and inflicting the Wet status on them. She will also cause a mark to appear on enemies that is proced by incoming attacks, increase their damage.
As you can see, Mona has strong crowd control and great setup potential for elemental synergies.
For Mona, a good build will center around Elemental Mastery and Energy Recharge over straight-up damage. We would suggest Mappa Mare as you can build it at the Blacksmith, making it very accessible and not making you reliant on Wishes for a solid build. It will increase Elemental Mastery, and will also provide a damage boost to elemental damage after you trigger an elemental reaction.
The Instructor Set, available throughout the game but at higher rarities in the Stormterror encounter, will give an Elemental Mastery increase of 80 at two pieces, and causing an elemental reaction will increase all party members’ Elementary Mastery by 120 for 8 seconds at four pieces. This makes Mona a very potent link in your character rotation chain during fights.
Mona can be an exceptional fit in the right party, but will work best with other ability spamming characters to really take advantage of all those abilities flying around.