Yes, the quest Earrings can only be equipped to Sakura and no one else. You can obtain it by completing her Character Quest.

If you're asking whether the dressblooms are a regular feature in the store, no, they're not. In fact, I think it's a once in a blue moon kinda thing. Ever since the release up until now, that was the first time it ever appeared. Btw, with more and more 5★ girls being added (seriously, they're gonna way outnumber those 4★ girls), you're not gonna get 5★ dupes that easily plus with that 6% chance.

Don't worry too much about getting dressblooms for your 6★ girls, coz obviously they're already OP. Just equip a ring with a high ATK and you're good to go.

Dressblooms were actually in the shop before, but those could only be bought with Nutaku gold. This is the first time they could be bought with FGs.