At first glance, Monica is a decisively average healer. That remains true on second glance: Her heal is competent, more powerful than some other SS-ranked healers, but not by a large margin and with potentially less side effects i.e. Aphrodite. However, she's well worth a third glance- Monica provides two advantages over most of her green competitors. First and most obviously, Monica offers a speed buff, and is one of the first cards to offer that. At SL7, with a small (15%) subcard skill powerup, this is a +2 boost and a quite credible boon to people interested in playing difficult to hit cards or simply interested in having some shots sail over their head.
More importantly, Monica is one of a very small handful of cards that spreads in a standardized manner- while some cards offer additional AOE effects, Monica completely duplicates her own effect to all targets. This means that Monica is also one of a small number of cards able to make full use of subs such as Cronos and Xebec, as on straight+ she will provide their subcard buff to all targets! If you have both of these cards, Monica can act at as an incredible mid-range healer- she heals significantly, though not astoundingly, and also offers a basic three-way buff, making her flexible and useful whether you simply want to pre-fight buff and top off before a boss or she's what you drew to save your team (as her heal also spreads) at the eleventh hour.
Without these, or if these cards are already allocated, Monica's value depends on how badly you want an AGI buff- she is without a doubt one of the better ways currently to get that buff, but if you have no interest in that, she's best passed over for Dryad (if you want pure AOE healing) or Aphrodite (if you want pure team buffing.)
Note that when fielding cards such as Cronos or Xebec, they will only apply to the party if you are able to play her in a straight+ value hand.
Monica's subskill isn't amazing, but it's without a doubt worth note! While not a standout, she makes a great sub for herself (being just enough to push her to a +2 agi buff at SL7) as so many cards do. More importantly, she can serve as a stand-in for any demon class- and every single demon, excepting Baphomet who serves a different purpose (but can still utilize this boost), is a very powerful card! This means if at any point you need to swap Monica out for someone else, she can plug in to one of the demons you'll likely have in your deck and offer a credible bonus still. While she may not be best in slot for any demon, she can plug in until you find a better sub.