I'm going to tell my fiance that I'm too tired to attend his daughter's ballet performance
Do you have a good reason for skipping the performance?
I don't feel like I'm in the mood to attend this kind of thing.
It's okay to take some time to yourself, and you don't need to attend all of your partner's kid's events. However, if you really have no interest in his child, you shouldn't lie about this being a temporary thing, as you and your fiance will need to talk about this.
What if I just skip the event and act like I didn't get his message telling me about it?
I think if you are having problems with your fiancee then you must talk to him. You shouldn't lie to your partner about things like this. This will be something you will have to do in the future anyway.