why is "thật "before "là"?
And why not just "thật hay"?
Would tốt work here instead of hay?
It's still okay. But we usually use hay to talk about our feeling about something while tốt for the quality of something.
That's why I thought it would fit here: We could be talking about the quality of the violin itself. If the sentence was 'She plays the violin very well' then 'hay' would seem to be the right one to use.