I've taken to more or less doing that. The next girls I want to evolve aren't any higher than level 40, but I'm running each of the relevant dragon maps 1-3x per week as it fits in. It's still annoying to get 5 rare chests and see 5 lvl 20 dragons.
Back to the event: I just hit my goal of 20k sweet potatoes after the event girl and all her gear - enough for all the manyus and all the attack and defense ampys. Chests during the event gave me a surplus of HP ampys already so I'll only buy those if I have a lot leftover from the next few days (and maybe some cake because the last two rainbows I got are both fond of cake). I'll pitch in for ally spawned event bosses still; I probably won't spawn any of my own (there were 11 on my list just a few minutes ago) unless there's a shortage of other people's bosses.