Was Hippomenes mortal?
Yes, Hippomenes was mortal. He was also known as Melanion. Hippomenes/Melanion was the husband of Atalanta.
When Atalanta broke her vow by marrying Hippomenes did she dishonor Artemis?
Most likely Artemis was mad though she did not punish her. It is not told in the story whether or not she was mad