The day of the chasana is a personal Yom Kippur for the chosson and kalla.
If you are talking about one of the guests rather than the kalla herself, that is also an inyan as the chuppa is supposed to be an eis ratzon.
Either way, davening is generally a good thing and you shouldn’t make fun of people who do it (unless of course they are doing it in a way that is disturbing others or at a time when they shouldn’t be for whatever reason). Better to daven during the chuppa then sit around speaking lashon hara.