How does Mona work?

Mona is mostly used as a CC spammer, meaning she depends on her elemental skill and burst a lot for her damage. For Mona, the most important thing is Stellaris Phantasm; The damage achievable by this skill is no joke, which is why you want to aim to have it available as much as possible, which is why we build Mona for Energy Recharge. Luckily, this does not only help her burst's uptime, but also increases her Hydro DMG through her second passive talent. We need to deeply understand how Stellaris Phantasm works to maximize Mona's potential.

Stellaris Phantasm

This skill is broken with the insane amount of damage and crowd control (through its big range) you can achieve with it, but you need to learn how to unlock its potential. This skill really shines with a pyro character to trigger Vaporize, but there's a catch. You need to know how Stellaris Phantasm works for this. On cast, this skill will engulf enemies in a "bubble", and apply Hydro the "Omen" status to them (which is a big source of damage bonus for attacks), giving a damage bonus to attacks. After some time, the bubble will automatically burst, dealing a large amount of Hydro damage (Do note that this damage is not affected by the Omen status damage bonus). During the duration of the burst, small enemies will be trapped in that bubble and be unable to move, while larger enemies will still have the "bubble" effect on them (the Omen status and the damage of the bubble burst) but will still be able to move. You can also burst the bubble yourself by hitting it, and the burst damage will happen right after the hit. There are two important things here to note for maximizing this skill's potential. First is that this skill applies Hydro on cast, and the bubble burst is also Hydro damage. This means that if you simply cast the burst, and switch to a Pyro character and use a skill to trigger Vaporize, the damage will be purely Hydro and the reaction will not happen.

Why does this happen? Remember, the burst applies Hydro. When you hit with your Pyro skill, Hydro is already on the enemy, which will react with your skill and trigger Vaporize, but also remember that the bubble burst damage happens right after the hit; By the time the burst damage happens, Pyro had already reacted with Hydro, and no element is currently applied on the enemy to react with since those elements cancel out each other on reaction.

What should we do then? Apply Pyro before casting the burst. This is so that the Hydro application of the cast will cancel out with the Pyro first. No elemental status will be on the enemy, and when you use your Pyro skill, you apply Pyro, and immediately after that the bubble bursts, and triggers Vaporize with its reaction damage.

There's also a catch here: Mona is the one triggering the elemental reaction, meaning that its damage will scale off her stats, not the Pyro character's. This means that you do not need a high level Pyro character at all, as a level 1 will do since all we need is Pyro application and since elemental reaction damage only scales off the stats of the character triggering the reaction. In addition, this is an ideal situation, because Vaporize is one of the special reactions that get a damage multiplier, and that multiplier is 2x if the reaction is triggered by the Hydro element, and 1.5x if triggered by the Pyro character. This is why we want Mona to be the one triggering Vaporize. Moreover, unlike other reactions beside Melt, Vaporize doesn't scale only off Elemental Mastery and character level; it accounts for ATK, CRIT, Elemental DMG bonuses etc, and all of these stats are obtainable through artifacts and weapons, which is why to maximize Mona's damage, we want to always be triggering Vaporize with her since it scales off many more factors than other reactions, hence more damage.

One more thing: To achieve the highest damage possible out of the bubble burst, you'll need to wait a little bit before bursting the bubble for the ICD of Mona's Hydro application.

TL;DR Apply Pyro, cast Stellaris Phantasm, wait ~3 seconds, use Pyro skill = Absurd Hydro damage.

What constellations are worth aiming for?

What are the chances of you having a Mona, or any 5-star constellation for that matter? Not very high. You don't have to fret about this. Mona does NOT need any constellations to be the insane support she is; In fact, for a support build, only her 1st, 3rd and 4th constellations are worth it (3rd for Stellaris Phantasm levels, 4th for CRIT Rate for other party members). Her first constellation is amazing, as it will increase the damage you deal with Vaporize through your burst on other enemies once you pop a bubble. You could also cast Mona's elemental skill right after you burst the bubbles, so that it constantly applies Hydro, then switch to other characters and trigger reactions.

If you are building Mona as a DPS character, only her 6th constellation really boosts her damage. Her second constellation is not worth it because Mona's normal attacks have bad scaling anyway, so you should be spamming charged attacks with her if you build her as DPS (In fact, this constellation effect has a 20% chance of being triggered, and during the time you try to trigger it with your low damage normal attacks, you might as well just use charged attacks instead).

Talent upgrades?

Whether you're building Mona as a DPS or a support, maxing out Stellaris Phantasm comes first (That's just how absurd the damage from this skill is). After that, if you're building her as a DPS, you want to level up Ripple of Fate (Your normal/charged attacks talent) then your elemental skill (Mirror Reflection of Doom). If you're building her as a support, level up Mirror Reflection of Doom, since as a support, you'll only be casting her cooldowns then switching to other characters.