No, you don't need a POA to use the account. But why are you on the account? If she still has access to the account won't she still mess it up? How will you prevent that just by being on the account?
You might consider taking away her ATM card and checks so those don't get used by her. Of course she could always get them replaced or use a bank counter check. Maybe you could just change the account to require both your signatures. That involves her participation in her financial affairs and is less likely to cause potential problems down the line if anyone is to question what you were doing with mom's money.