Dandy Guy, in Space
As IncendiaryLemon says, it isn't needed in order to understand the film and I think it holds up pretty good on its own. That being said, if you pay close attention you can deduce certain things regarding the events in the two seasons of Sound! Euphonium (so in that sense it "spoils" certain things a bit).Is it necessary to watch Sound Euphonium to understand what Liz and the Blue Bird is about?
So if you can, I would still suggest watching Sound! season 1 and 2 first and then the movie. They're all great works and well worth a watch if you ask me. Liz and the Blue Bird in particular managed to exceed my expectations.