Surprisingly good for a healing card released A.C. (anno custodi, in the year of Warden of Healing).
For starters, Ioreth is free to play. Granted, she's not free to activate, but the fact that she's free to play means that you can only invest in healing when you need it, as opposed to Warden of Healing who you need to spend 2 resources on in order to have access to a card ability that you may turn out not to need over the course of a game.
As for that card ability, Ioreth occupies a different niche than Warden of Healing, providing a strong burst of healing to one target as opposed to weak healing to multiple. Now, this does mean that Warden of Healing is better than her in some cases, such as a quest where there are effects that spread damage as opposed to clustering it, but it also means that Ioreth is actually better than Warden of Healing (wow, never thought I'd hear those words) when you're more focused on keeping 1 character alive, such as a Treebeard deck where Warden of Healing's spread effect is less useful because only 1 character is consistently taking damage. Of course, the fact that Ioreth only targets one character also means that Elrond is less effective paired up with her, but that's relatively minor and says more about Warden of Healing.
Now, Ioreth is expensive to use consistently, but you don't necessarily need to use her consistently (and the decks that do should be bringing plenty of resources to fuel her and Warden of Healing, i.e.; Treebeard and Glóin). This, combined with the fact that since Ioreth heals 3 you can relatively safely stack damage on 1 character before healing it all off at once means that Ioreth is actually cheaper than she looks. Granted, those turns that you don't need her she does nothing, not even having 1 printed and unable to attack or defend, but Warden of Healing's the same case, as typically you don't know whether or not you'll need healing until after the quest phase, when you could conceivably put his 1 to use. You can conceivably boost her with Faramir or Visionary Leadership, but I'd argue it's better and safer to simply hold her back and accept that the ally that you paid 0 resources for is going to do nothing some rounds.
Overall, I rate Ioreth a N.W.T.W.; Not Worse Than Warden. Her lack of upfront cost and burst healing make her a viable alternative to Warden of Healing in decks that don't intend on seeing a lot of characters damaged and her ness means that, so long as you have more copies in hand or your deck, you can relatively safely lose her, as you can simply play another copy and experience no loss to your maximum board state.
To be clear, Ioreth does not surpass Warden of Healing. And that's a good thing, because I'm pretty sure that Warden of Healing getting upstaged is up there with the angels blowing the horns and the Four Horsemen as signals of the Apocalypse.