I feel that people are missing the forest for the trees when saying "There's no good grass pokemon in rotation".
If you played in Welder format you would know that none of the attackers had particularly powerful effects, which also meant that once Welder rotated most fire pokemon went from "serviceable" to "wow this sucks without energy accel". Hell, the deck that won worlds was just a collection of powerful and versatile attacks + Mewthree + Welder. So I'm gonna thinker with Gardenia for the meanwhile before dismissing or calling it "broken" - though my TCG experience tells me the balance can easily flip to the latter.
However, I personally can't see Mew decks switching to Gardenia as a supporter of choice. The deck gets its blazingly fast consistency from being able to run very little energy cards, and you can already play Double Turbo for the attack on a single attachment on Mew without running headfirst into the issue of having to get to energy cards without compromising consistency, and then having space for the rest of your engine.
Fire mons were good. Blace could hit 300. ReshiZard had 3 useful attacks in total. For a time, Charizard gx existed. This isn't as effective a draw engine and requires switching cards to be able to be used, and has way worse choices in terms of attackers. Not saying it cant be good, but I doubt it. This card could still be useful as a general energy accel card esp with dte, but I dont know off the top of my head who loves that that is also consistent.