What is the origin of the French designation "MOMO" for Muhammad and "MOHAMET" for the Prophet, and does it contain any offense to our Prophet Muhammad, peace and blessings be upon him?

It is permissible to use a diminutive form of a name for endearment, but not for belittlement, based on the Almighty's saying: "O you who have believed, let not a people ridicule [another] people." If the intention is to belittle the Prophet, peace and blessings be upon him, then this is an act of disbelief. It is permissible to use a diminutive form for the purpose of endearment (tarkhim). As for pronouncing the name "Muhammad" with a 'taa' instead of a 'daal,' it appears to be a pronunciation style of some non-Arabs, and it is not considered an offense unless the speaker intends it as such.