i think if mariah were to do a reality show, the direction of the show she chose to go for would determine the network it would be aired on.
for example, if it were to focus more on her music and her recording her upcoming album etc. then mtv/vh1 for sure since they're music oriented (for the most part, aside from the sh!teous shows like jersey shore, teen mom etc.)
but if the show was going to focus on her new role as a mom then one of the "lifestyle" networks like tlc/bravo would be better suited for that.