Hello Chris!
Yes, it is permitted to alter any track made public (or not, but that would be considered stealing in real life). There is no intellectual property right to UGC, Ubisoft is the sole owner of the content created within the games using the game's services (here, Trackmania). Hence, other users can, as well, modify, copy and share others' creations.
However, we would take action if someone would repeatedly steal someone's creation and take all the credits in Track of the Day for example. But that decision will ultimately come from us only.
Cheers!
Darrek