Welcome to the performing arts world.
To be blunt, there are some kids who are naturally good, work hard and just have that x factor, there are others who are naturally good, work hard but don't have that. There are thousands who have a lot of fun and enjoy it - but no matter how hard they work, they are never going to be stand out. And that is also fine.
The important thing is how you manage it.
My DD was in high level training from a very young age - she's a good dancer and a stand-out singer. She was never in the same league as the really good dance girls she trained with - and never would be, because she lacks that X factor that some of them had. Even among the best, there were one or two that were just lovely to watch even if they weren't the hardest working or even technically 'the best'.
It did dent DD's confidence. She eventually dropped dance for a year, then picked it up again at school - where she was one of the standouts, as the level was much lower and she was very well trained. She knows full well that it hasn't suddenly made her an amazing dancer - but she's rediscovered her enjoyment and teachers were giving her chances to work on designing choreo and being front centre for the first time. She now does dance purely for fun and exercise.
Six is probably a little bit young to know for definite as some children do suddenly grow into things, but you might be best to keep your thoughts to yourself for now, but think about a less competitive and more inclusive school and a more 'for fun' environment in the future if you find her confidence is being knocked. Both types of school have their place.