Why would a judge think it would be a good idea to make a repeat offender a foster parent?
Because although Madea is repeat offender, the judge knows her well enough to know that any child that goes into her house turns out well-behaved and respectful of others. Plus, it was more of a way to get back at Madea than actually thinking it was a good idea.
I saw it as a "last resort" thing as Nikki's other foster guardians she either ran away from or no other foster guardians would take her. Really, Mablean was sick of seeing her and Nikki.