Nope.

MPM fares can go anywhere so long as they meet the other fare rules like number of transfers and such. That's part of the fun with them. That said, staying under MPM without mostly just going through the hubs is generally uncommon.

Also worth noting that some MPM fares allow a surcharge for exceeding the distance. So going over by 25% could see a 25% fare increase, for example. This was commonly useful a few years ago on AA MPM fares from the west coast to Asia as a "backtracking" routing via Miami was relatively cheap to add, even exceeding the MPM. Especially because the YQ/YR doesn't get increased, just the base fare. So a super cheap base fare where most of the cost was buried in the YQ was an even better deal if you wanted to fly crazy routings.