No, MXM is the form factor, but there are many revisions. The TDP allowed is set by the amount of power the board is designed to deliver and the heat sink of the laptop is designed to accommodate the dissipation.
If you have an existing card in the slot, find out what its TDP is, that should be your rough maximum.
If you can find the MXM revision number, then you can check here for the maximum TDP. Since these are guidelines, OEMs may do tricky things like undervolt/underclock a card so that its TDP is within spec.
https
/en.wikipedia.org/wiki/Mobile_PCI_Express_Module