One has to keep in mind that all Trump's doctors are publicly giving Trump excellent bill of health for years.
The people cannot even get their hands on Trump's tax records. Much less his health records. So asking about diapers is an indirect health check.
Like asking why a billionaire had to use a bail bondsman for a $200,000 bail.
Nobody cared that Trump was wearing a diaper until he acted like the emperor with no clothes.
Nobody usually cares that the President got experimental drugs. But when Trump got Covid-19, he was pushing for Hydroxychloroquine based on questionable sources.
Part of politics is the ability to take jokes. Biden attended the WH correspondents dinner. Trump sent his press secretary.