Is my aunt wasn't almost in tears when I dropped her wedding cake on the floor a understatement?

I assume you meant was*, not wasn't ... And that would most likely be an understatement. I can't imagine how upset someone would be if you dropped their wedding cake on the floor. Then again, the answer to this question is rather subjective. Different people would react in different ways if they were in this situation.

No itd be an understatement if she were to be "annoyed"