What I'd do is ring the college and ask - there are some rather outré skirts worn on Matriculation Day (last year we had puffballs and LOTS of miniskirts...), but everyone I saw was wearing a pretty standad blouse. If you really have your heart set on wearing it (and think you could successfully tie the ribbon!), then just take that shirt AND a "normal" one so that if a member of College staff sees it (before you set off to the Sheldonian) and says it's unsuitable, you can run very quickly upstairs and change.