Can a judge overrule some poor decisions of POA?

A POA is a fiduciary, legally responsible under the law to make decisions for and in the best interest of a party when they can no longer make their own decisions. Who is questioning the decisions made by the POA? Is this a case before the court now? If so, then yes, absolutely. A judge will make the decision whether or not the POA IS in fact acting in the best interests of the person in question. If the Judge feels there is malfeasance, then the Judge can appoint a temporary of permanent guardian who will replace the POA.
More information from you might result in a bit better of an answer, but offhand these are the facts.

This is a problem with POA. When the person is not making decisions that are good for the principle. Or, taking money. There really is no recourse. If the principle passes and the Executor is a different person, than the Executor can request an accounting. Even if the POA is the Executor the beneficiaries can ask for an accounting. But really hard to prove anything before the person dies unless ur willing to sue for guardianship. If you win, you maybe able to recoup the cost of the guardianship from the principles money. I think Medicaid allows for that.

What are those poor decisions? How did a judge get involved? Were these "poor" decisions in the best interest of the principal or the POA? A POA's decisions can always be scrutinized by others and are not infallible. It's very important to designate someone the principal trusts and who volunteers to be the POA. If we're talking medical POA then it's also very important for the principal to have created a living will so the POA knows the wishes of their LO and can act accordingly.

I'm not sure you can take it to court unless you intend to sue for guardianship. You will need to consult an attorney and you will need to provide actual proof, and you will need to pay for all of it, knowing you may lose. You can start by investing in a consultation with an attorney experienced in prosecuting elder abuse. They won't move forward unless they believe they have a winnable case. Probably will cost you about $300-$500 and again, you need to present the concerns and proof.