It’s correct either way. As in English (“Are you sad?”, not “*You are sad?”), inversion is usually used in questions, but it’s not required in Spanish. Actually, you would normally omit the pronoun tú completely, except for emphasis (perhaps at the end for even more emphasis “¿Estás triste tú?”, and just say “¿estás triste?” This works well because the final S indicates the person. Note that in places where the final S is very soft, especially the Dominican Republic, Puerto Rico, and Cuba, it’s more common not to use inversion, so they would tend to say “¿tú estás triste?”

la forma correcta sería ¿tú estás triste? , pero normalmente cuando le preguntas a alguien directamente se omite el sujeto y seria algo como: "¿estás triste?", solo aplica en pronombres en segunda persona osea tu, ustedes, usted.