Not really the case, says Snopes.com:
The nuptial couple has announced its desire to hold a ‘people’s wedding’
populated with invitees garnered from the Prince’s work among his 21 charities
rather than one with a guest list adorned with heads of state… There’s no evidence for the tabloid-style claims that Prince William or
Kate Middleton have taken a dislike to Michelle Obama, that either of
them fear Mrs. Obama will attempt to upstage the bride, or that Mrs. Obama called Diana (William’s mother) an ‘over-sexed clothes horse’ or maintained she was ‘more popular than Diana ever was.’
Thank goodness. See the full Snopes story.