Is it disliked for a wife to call her husband Muhammad by a pet name derived from it?

There is no harm in softening, endearing, and making the call beloved; because it is a well-known style among Arabs, and this includes truncating the vocative noun by omitting one or more of its letters. The Prophet (peace be upon him) called Aisha "Ya 'Aish," and Al-Nawawi permitted truncating a name as long as it does not cause offense.