As someone much older, I'm a bit surprised that someone in their 20s likes that tunic - doesn't seem the sort of thing my DS's (child-free) female friends wear. Think you could choose something more youthful, but if you like it go for it. As you did ask for opinions, the tie-back to me makes it rather more frumpy, not less - probably because a lot of plus-size things have a tie-back. And it does remind me of Per Una, which is full of 'vibrant' colours.
I always like to give a bit of thought to whether what I'm wearing is generally appropriate, so guess I must have done that for DS's sports days - but don't remember it being a big issue. Usually too busy making sure I got away from work in time. Do recall spending time on choosing nice outfit for parents' parties, though.
As for 'dressing up' for the theatre, have to agree with a previous poster - to me, that is uncool. But if it enhances anyone's enjoyment, fine.