@TeddynMousey Ignore anybody telling your DD that it's 'inappropriate', what is appropriate for a 21-year-old celebrating 3 years of hard study is whatever she bloody well wants to wear. And no it doesn't have to look like she's going for an interview or starting a new job because she isn't.
I also have recent experience of attending LSHTM graduation ceremonies, which are held at a prestigious venue in central London, and would 100% endorse what @faffadoodledo says, that your DD's dress would place her firmly in the middle of what the women wore.
The only advice on here worth listening to are all those sensible people questioning whether your DD's choice of dress will work well under a heavyish gown. So check out the practicalities and ignore the value judgments.