The problem with cherrim’s flower gift ability is that it boosts the wrong stats. Imagine if cherrim had its special attack and speed state boosted in the sun! Not necessarily doubled, perhaps just buffed by 1.5. Cherrim doesn’t exactly have the best special movepool, but it does have STAB grass attacks + weather ball, which would make it a pretty competent sun sweeper.
Mmm, sure, sounds fun. I mean, the problem with reliance on weather as a general thing is that Tyranitar still exists and can say “nope” to you at will, especially if you have as much riding on the weather as Cherrim does. Also Cherrim’s starting attributes are poor enough that something like Chlorophyll Venusaur (who’s tough enough for Growth to be viable some of the time) might still outclass her. But yeah, Grass + Fire has always been great, and this would probably make Cherrim suck less, though I wonder if it actually decreases her flexibility in doubles, since much of the point of Flower Gift is that it applies to your whole team, and you now have fewer partners who are eligible for the damage boost. Then again, everyone likes speed.