You will need to be more specific than that.
Do you mean incompetence in the sense of being incapacitate due to a mental illness, disease, advanced age, Alzheimer, permanent, temporary, the inability of a person to make responsible decisions regard their person, their finances?
An individual can be incompetent mentally and/or physically in a medical sense and yet be competent to make a valid will. If that is what you have on your mind.
Generally speaking if the testator is aware of the nature of his property and the people that would normally be the object of his bequests, the will stands even though he or she is unable to make responsible decisions otherwise.
It takes a pretty strong showing of incompetence with clear and convincing evidence, before a court can deny the admission of the testator’s will to probate.
They don’t have to be able to read and quote from the Wall Street Journal.
And the court will NOT require medical evidence that the testator underwent a mental evaluation or “testing” before executing the will.
And post mortem testing for competency to make the will is difficult at best.
Is it possible that someone's will did not meet someone's expectations?