The crowd laughed and clapped as Ellen ran to her place backstage.
Did the crowd laugh because of something she had done in her performance?
Yes, she was really terrible. They were laughing at her, not with her.
It's good to clap for someone on stage, because this shows appreciation for a good job that they have done.  However, if she had done something wrong on stage, or messed something up, it would have been rude to laugh at her, because this is insulting, and she would already have been embarrassed.
If she had given a better performance we wouldn't have laughed at her.
It's wrong to clap mockingly at people, and laugh at them when they fail. I think next time someone has a poor performance you shouldn't be so mean about it.