The informant heard this joke from his uncle, a doctor:

An apple a day keeps the doctor away,

But if the doctor’s cute keep the apple away.

The informant points out that this joke can be construed in different ways, and that his uncle (the doctor) would say this joke in a variety of contexts.  He would say this joke to the younger people and kids, and then again to the adults, with different reactions and meanings.  The informant believed his uncle thought being a doctor made him successful and attractive, and this joke reflects that quality.

I agree with this interpretation.