Oneiromancy stole the show when it came out, taking the glory away from cards like Matta and Decree. Although I agree that Royal Decree is extremely powercrept (a 1 prov buff would help make it relevant again), I think that Matta Hu'uri is actually decent, under certain circumstances.
Matta Hu'uri draws the highest prov card from your deck for you, and draws the lowest for the opponent. Her 6 body usually matches the points of the card drawn by your opponent, meaning that she eventually acts as a 9prov tutor for the best card in your deck, while adding an extra turn to the game.
Since Decree is too bad to be worth mentioning, lets compare Matta to Oneiro.
Matta saves you 4 provisions over Oneiro. Oneiro often costs more than the high end golds it is trying to pull.
Matta helps engine heavy decks, by extending round length.
Matta can mess with adrenaline sequencing, either delaying or extending adrenaline conditions.
Matta gives your opponent a 4p card, which may play for less than its intended value if drawn late in the round.
Matta works better for cards with initiative. Regis is currently a meme, but Igni and Scorch are playable imo.
Matta puts the card in hand, allowing you to play it in a later round, and protects you from shenanigans like Ihuarraquax and Cantarella.
If your opponent plays Warrit, Matta allows you to draw the card without playing it, possibly avoiding overcommitment.
Oneiro tutors two cards; Matta only tutors one.
Oneiro pulls whatever card you need, making it great for pulling tech cards.
Oneiro can be used in a R2 drypass to pull a bad card from your deck.
Oneiromancy gives you extremely precise control over tempo.
Matta pulls can be random if there is a tie in provisions.
It's possible to absentmindedly play Matta when your opponent has 10 cards, thus bricking it.
All told, I don't think that Matta is or will ever be a replacement for Oneiromancy. However, under certain circumstances you might want to give her a go. Specifically, if you are only trying to draw 1-2 key gold cards and most of your bronzes/midrangey golds are interchangeable she might be better than Oneiro. 4 provisions allows you to upgrade a 4-5p bronze into a 8-9p gold, which is quite a swing! I suggest combining her with Maxii Van Derekar, so that you can know which high end golds you should try to draw at the start of rounds 2-3 and which need to be pulled with Matta.
In short I think you should conside her as an alternativeto Oneiromancy when deckbuilding.