Why is Prince William banging on about mental health issues, is he suffering from being a loon?
He and his wife and Prince Harry have made mental health issues one of their causes. That's been the case for several years now. That's why he's "banging on" about them.
Royal Family members support MANY good causes. Having them as patrons/mouthpieces helps publicity and fundraising. You have a problem with that?
He and his wife are being supportive of people with mental health problems. Millions of people have such problems, to a greater or lesser degree in their lives and like many members of the royal family, he's supporting charities and awareness. Do you really think that it's 'being a loon' to either be charitable and supportive or to have a mental issue?
Mental health can be a big deal for a lot of people, it doesn't mean you are crazy just because you have a few mental health issues.
All the royal family are loons so he's talking about his own.