It was my impression that she was so shitty to him because of Boimler’s little slip up. She definitely could have been kinder, but she knows she isn’t even a blip in Spock’s life. I think she feels she might as well move onto something where she can make an impact and be remembered, like her career. She is probably bitter, and it came out that way as we are all so uninhibited when we spontaneously break into song.