Thank you for your query. It depends on what you mean by improvement of the nasio-labial lines.
If the nasio-labial line is deepened because there is lack of support from your smile and your teeth are collapsed inwards, then braces may help improve these lines. Braces can be used to straighten your teeth and improve your smile arc which 'should' improve the naso-labial lines. I use the word 'should' as this is very variable between patients.
However if your naso-labial lines are not due to collapse of the teeth then braces will not help. I will normally refer these to my aesthetic medicine colleagues.
I hope this helps. Good luck!