Yes, Serena made a mistake.
I don't like Blair b/c of how snooty she is. She thinks she's better than everyone, puts her nose up to everyone, she thinks she is soooo innocent, and always the victim. She is manipulative, and sneaky. I don't like her character. Serena herself, yes, she made a huge mistake, and then went further to not be honest with Blair. I am not condoning what she did. But, all in all, she comes across as a better person. She is courteous, and nice, and giving. She is upfront and honest, and isn't trying to trick those that are close to her. I mean, Jenny has been great to Blair, and she wants to go and get her in trouble for breaking into a store??? How rude and distasteful is that???