The deal would be like MM would just stir up fake trouble, like busting into a bank or hijacking a train. Mr. Incredible would show up and stop him. Mr. Incredible gets to feel like a hero, MM gets someone to challenge him, no one is hurt, and it repeats. MM would probably prey on Mr. Incredible's desire to want to be a positive influence and probably tell him that his actions would help inspire other citizens or something. That people need hope just as much as they need heroes.
Mr. Incredible agrees to it under the condition no one gets hurts and it works out for awhile until MM starts to feel bored and ups the ante a bit or maybe begins to feel belittled during Mr. Incredible's heroic bants.
Anyway, it's just an idea.