The battery pack is sealed and is up tight against the cabin floor so there's probably no place for a mouse to access for warmth. Charging at the maximum AC power of 7.2 kW (30 A @ 240 V) isn't likely to warm the battery pack much. If you have overnight to charge, you could reduce the charging power which would reduce the warming yet should still allow a full charge by morning.
A few i3 owners have reported rodents having chewed through wiring which can be an expensive repair. I don't recall anyone assuming that the rodents were attracted to a warm battery pack. This problem doesn't seem any worse with an i3 than with any other car, maybe even less because there's no warm ICE to attract rodents in cold weather.