How to make my mouse stink less?

There are a number of ways to control the odor. The most preferable of which is to regularly change the bedding to reduce chances of the stink of the urine to gain up over time. Change your bedding at least twice or thrice a week to control the odor.

The type of bedding also contributes to the smell your pet can create. Aspen as bedding is fine, however I recommend that you use corncobs. There's a research that concludes that using corncobs reduces the chances of ammonia building up in your mouse' cage thus reducing the smell.

You can also spay your mouse, which is a sure fire way to reduce the odor. However, spaying simply to control the odor isn't recommended as the surgery carries a significant risk for your pet.

Use scented kitty litter under the bedding.

put it outside.