I don't blame you for being upset. She obviously wasn't joking because you are not watching Dumbo. But don't let it get to you. Just let it roll off and move on with life. You and your family know Mikey is not mean. And we all know it here too.
It is sad that you won't get to watch Dumbo. But maybe someday she will see that Mikey is a nice pup and all will be well.