I got a -Atk + Hp Winter Tharja. How bad is this?

Remember, with Vengeful Fighter she's going to be triggering Special nearly every turn, not to mention attacking twice both times. Don't sweat it, IV's aren't life and death :)

It shouldn't matter too much. It may depend on how you build her; with a blade tome for instance, she can more than make up for that with armor buffs.

I always check kagerochart whenever I want to check how a unit does against other units. You can check a unit versus all units, either attacking or defending, or do a specific matchup. If you do all vs defender or attacker vs all, it'll just be the base kit, though. All that aside, your Tharja with base kit still kills 138 units, loses to 24, and draws 37.