Do I need POA even though we have joint account?

Hopefully you a just a signer and not joint, because the bouncing checks will show up on your credit history if it is joint.

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.

I supervise her bill paying. If she doesn't pay something I do. We've had a talk s she knows that I will be taking over soon and paying her bills.

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?