More particularly ..
(a) Vmo and Vne are different animals (
FAR 23.1505)
(b) Vmo will be limiting at lower pressure heights, Mmo at higher
(c) for Part 23 turboprops, it is common to express the POH limitation in terms of Vmo .. and then adjust Vmo for prop limits (generally at a constant M) rather than call it Mmo explicitly/separately .. eg, I have a KingAir manual to hand and the wordiology is along the lines of "Vmo to FL210 is 260KCAS reducing FL210-FL350 260-192KCAS" The limitation notes that the reducing limit is at 0.58M and there is a reference to the barber pole's reflecting Vmo/Mmo limits.
(d) reference to the FAR and typical POHs will find the usual "These speeds may not be deliberately exceeded in any flight regime" (eg the KingAir manual to hand) or the limitation with the FAR permission "unless a higher speed is authorized for flight test or pilot training " as appropriate to the particular Type. It is understood that unintentional, minor, short term exceedances due minor turbulence (usually) or at the crossover height on descent (if asleep) are part of the normal gameplan.
(e)
It's a GOAL not a LIMIT .. quaint view .. I incline to 411A's observation ...
(f)
dived you’re [sic] a/c beyond Mmo ... likewise ... in so doing one would be well in excess of the Mmo limit ?