[edit: That's funny, I think the post above yours was your post, now that I look at it again.]
The post above yours is mine, the post above that one is not. I spent only 3 tries to get 2 costumes, it was easy. Then spent 2 more tries on next costume, the game became much harder and so failed both tries, last one at the final moment while missing the huge egg on 10th level. That's when I got annoyed and wrote a whine )

The post above mine also talks about two costumes and that it became harder after that, what I don't know now is if it becomes even harder after you get more costumes. Will try to do more today and check it.

I got one for Cherry (bad looking one) when I just tried the event and one for Maggie, this one looks kinda funny, pink, without egg basket and with sparkling nipples, also it is first 70% for her, so that's good.

I agree that there is no reason to collect costumes, they just stay in storage and you don't see them at all. If they would give at least some passive bonus for owning them, maybe it would be more interesting to collect them. 1% extra income per each event costume that you own, for example. Or extra multiplier modifier.