She's already an unconventional leader, and as unreliable as Euron is she could have chosen any random follower. Marry the first son of a house with a lot of farmland, marry a slave master from Quarth, marry dude from the Golden Company.

Marry somebody who's bringing something to the table, not the the man that killed his father, kidnapped his sister, and wants nothing from you but the throne.

Obviously Cersi has always believed in the power of her pussy, but her fucking with Euron is short sighted and not inline with what Tywin would have done.