Doesn't seem that bad to me, the virus doesn't need Molnupiravir to mutate. It can do very well by itself, in the billion people and animals it infected. The fact we are seeing convergent evolution shows that it is naturally very good at this.
By increasing the mutation rate way beyond the sweet spot (that what's Molnupiravir does) the virus may get some desirable mutations, but won't be able to hold on to them. And if, thanks to the drug, the patient is cured, it is bad news for the virus, those who are not infected can't produce new variants.
But like antibiotics, I think this is an "all or nothing" drug. Either give maximum dose to do as much damage as possible, or don't give anything at all. Too small a dose may promote building resistance instead.