While there are surely multiple factors involved, the biggest difference I see is that Donald is a common name but Elon is an unusual one. Almost everyone has known multiple people named Donald in their personal life, as well as other public figures with that first name besides Donald Trump. Most people don't know any other Elons besides Elon Musk. Thus, the mental association of the first name with the individual in question is much stronger with Elon Musk than Donald Trump.

If someone said they were considering naming their son Rush, then most Americans would immediately think of Rush Limbaugh, but if the same person said they were considering the name Sean, I doubt the majority of people would immediately think of Sean Hannity, as Sean is a common name while Rush isn't. Common names have less risk of guilt by association, in a sense.