Yes squirrels are pretty active during winter months and even during snow.
If you have a bird feeder you'll most likely notice that squirrels will still be out eating or trying to get at your bird feeder even during snow.
Although you'll usually see less squirrels out during the snow you'll still see some squirrels out even during the snow and cold weather.
Although the squirrels are out mostly before fall and winter gathering food to save for winter they will still come out during winter and do not hibernate.