This is my theory: Iruma becoming a kingmaker shows that he can be a great king for the underground in general, since he has to delegate jobs to the rest of the demons. The Demon King's word is law, and so, he has to be insightful enough to know 'Oh, someone else would be better at doing this task.'
A true king keeps his subjects happy by giving them a bit of power, but still keeps them in check.
I think Mephisto's objective of making Iruma a Kingmaker is a completely benevolent one even if self-interested, it's almost as if he has been waiting for someone worthy to follow and bet on to appear. This test is a reflection of Mephistos itself. He likes the idea of helping someone become king, supporting from the shadows. Mephistos is playing Kingmaker both literally and metaphorically with Iruma, and also making him play the same to see if he can think similarly to himself.
This is double layered test, essentially.