When writing a screenplay, is it appropriate to describe the manner in which a character delivers a line?

That's really the Director's job, to interpret how the character/actor should express the line. You can put an OCCASIONAL comment like you show here, but don't overdo it. Some Directors are offended by too much "help" from the screenwriter, who by the way is seen as the lowest on the food chain in the film industry anyway! Just be careful!