the Kirby 'bag' bedtime shirt is just that, a bed time shirt / pajamas.
If you scroll down that link page there are some fairly standard lingerie pieces that simple have 'kirby' written in little letters on them and don't like like Kirby at all (beyond pink).
its really not that weird. The first item people are commenting about is a pajama shirt, its the same thing as a chick wearing a giant oversized male shirt as a bed time clothing article.