It's fine to adopt a dog during any season! There are dogs that desperately need homes all year round. That said, if you are housebreaking a new puppy, it can be a little more cumbersome to have to repeatedly go outside with them during the winter months. But lots of people adopt during winter, so you shouldn't let that stop you. (Also, you can avoid the housebreaking thing altogether if you adopt an adult dog who has already been house-trained.)
Here are some tips from the ASPCA website on adopting:
https://www.aspca.org/adopt-pet/adoption-tips Adopting a dog is a big responsibility, so make sure you are emotionally and financially prepared for that. But it is so, SO worth it. Good luck!!