Mice have always been thought of as pests, but as generally more sophisticated pests than ants or spiders or what have you. They operate in different ways. Ants come in droves and ruin everything you have, not scared and will not hide from you.
Dreaming of dead mice means that you have shaken some silly fears that you have had in your life recently. You no longer have to worry about these small little annoyances that have been following you around your entire life. There are a lot of little things that scare you about right now, about the future, about your own plans for the future and what you are supposed to do, but a dream like this means that you have finally figured things out just a little bit. You are finally able to make sure that you do not have to worry or fear about the future anymore. You are free from worry and can relax a lot more than you had been up to this point.
Mice are also known for being very timid creatures and being frightened of just about everything, entirely skittish. Therefore if you have a dream about dead mice then this could mean that you've gotten over a lot of silly fears that had been plaguing you for a long time. This is a good thing overall.