Her rainbow status is more to celebrate the fact that she's the most popular 3*. A free rainbow obviously won't compete with an actual rainbow. She's more akin to a gold (and slightly weaker than Helenium).
Even then though, she's in the upper tier of golds as far as TP. She's just below the Cosmos cousins and Star Lily, but above Evening Primrose and Edelweiss.
Also, assuming we get the other rewards, this is a rainbow you can actually max skills/equips without blooms. I'm pretty sure you get copies of her that upgrade her 100% or something. I think there's one such reward for hitting level 150, but I'm not sure of the other two equip slots. Maybe level 100 too?