Yes. The fact that the person making the Will has weakened mentality sometime after the Will is made has no bearing. It only becomes important should the person having an unstable mind want to change the Will at a later date. However, capacity may be generally defined as the ability to understand that a will is being made and to understand its general nature.  The fact that a person does not fully understand the full meaning and all ramifications of a will does not mean that the person lacks capacity.