Any company judging "maturity" of actions done by someone in their spare time is not worth working for. It's just not their business to decide whether someone is "mature" or not, so if they cannot manage their business properly, why should I want to work with them? They probably won't be able to maintain healthy relationship with their employees anyway if they're so keen to filter out perfectly fine candidates over something so unimportant and nonharmful as low-grade sense of humor.
But if you maintain a good wall between your professional life and that immature spare time, such that the employer doesn't even see the low-grade stuff, then fine.
It's hardly even "acting unprofessionally". It's a lighthearted project where, well, some part of the fun is waiting for the first person to draw some dicks in. If you can't get over it, that's more likely problem with your attitude, not the person's who drew it.
Anyway, let the mutual filtering continue. You don't want to work for a company that cares about drawing public dicks, and such a company won't want to hire you. It doesn't really even matter to decide whose attitude has a problem -- you're just culturally incompatible.