This game is not "bad" as in it has critical flaws, it's bad as in it's just very uninspired (and uninspiring) I should say.
The marshmallows (wait, they're not raindrops even though the game is called rain drops, are they?) fall naturally after some time, but besides changing color there seems to be no other indication of exactly when.
Mission mode makes you do different missions, but all of them can be summarized to one line: "Match as many as possible"
You can buy usable items and support items, the cost is pretty high in early game to spam them. I spent 500 on "increase money" support item but doesn't seem to be doing much if at all (again, early game. It says effect increases as you progress).
Overall, meh for people who like challenging puzzles. This is more of a mini-game than a puzzle really.