Magic Mouse battery level broken in Lion?
You are not the only one. Mine says the same - 100%. This is a bug. The terminal command for querying the battery percentage (probably what iStat Pro uses) reports: "BatteryPercent" = 82

For bluetooth mouse:

ioreg -n "BNBMouseDevice" | grep -i "batterypercent"


For Apple bluetooth keyboard:

ioreg -n "AppleBluetoothHIDKeyboard" | grep -i "batterypercent"


I've posted this in the Apple Bug reporter. Hopefully it will be fixed in an future Lion update.

Here is another thread discussing the same issue.
https://discussions.apple.com/thread/3211868