Grass was never my favourite type, and indeed was a decidedly uncool type back in my youth. Ain't no-one winning cool points for picking Bulbasaur or Chikorita as their starter (that changed a very little bit from Treecko onwards, though).
I don't think it's helped by the fact we've had so few Grass-type legendaries - by which I mean purely Grass-type legendaries. Part of me thinks this is because pure Grass is generally considered a fairly underwhelming type, so whenever we do get a legendary of that typing it almost has to be dual-typed to compensate - pure Grass struggles against Dragon, Flying, and Fire, all of which are relatively common typings for other legendaries.
We have obviously had a handful of Grass-type legendaries already, but - and this is only my view - for most of them, their Grass-typing often plays second fiddle to their other type. Tapu Bulu and Virizion are part of larger groups, and so the Fairy element is the core aspect that links all four Tapus while the Fighting element is the commonality between the Swords of Justice. That's not to say Grass isn't central to their design, especially in Tapu Bulu's case - but it is, ultimately, secondary.
I like Calyrex a lot, and its Grass-type is certainly important to it as it plays strongly into the harvest legend around it, but its Psychic-typing ultimately has a greater prominence - and it's the Grass-type it loses when it merges with its steeds.
There is, of course, Shaymin! But it gains a secondary type when transforming into the objectively better Sky Forme, so the pure Grass-type it has in Land Forme ends up feeling like a starting point instead of its own thing.
I almost forgot to include Zarude on this list, which I think is quite telling. But... yeah, Zarude exists. Nice, isn't it?
Likewise Wo-Chien.
I'll finish on Celebi, which is probably my favourite mythical overall for its lore and general uniqueness. Celebi actually does go in pretty hard on its Grass-typing - it's concerned with the preservation and protection of old forests, with plants and grassland. It's one of a few legendaries where the Psychic-typing feels a bit tacked-on, as IMO it's to justify it having time-travel powers. Like I think Grass/Psychic was a pretty great typing for Celebi and definitely helps in some regards but I think it absolutely could have been pure Grass and it would have worked fine conceptually.
But it, unlike Shaymin Land Forme, is pretty badass. I've always liked it a lot.
All this is to say that I wish we could get a pure Grass-type legendary as part of a game mascot trio/duo, instead of the lower trios or a mythical. Something Kyogre-level, unashamedly Grass and nothing else - and, crucially, strong. Impressive and fearsome and cool enough to make people want to use it. I actually always thought a legendary mascot trio of pure Fire/Water/Grass would be neat, as it'd be a neat parallel to the starter trio. Sadly, such a thing has never materialised.