There are plenty of ways for Ministers to be bad.
A few muddle their private interests with their public duties. They claim costs they should not claim, pursue causes and decisions for people and businesses that have supported them, receive income and gifts they do not properly account for or should not have received. In extreme cases they commit crimes.
Many play politics instead of managing their departments and acting in the public interest. They think the way to deal with a problem in their department is to blame someone else, help a cover up or shunt the issue off to another Minister or department. If someone has a real problem the department could or should sort the Minister should not contribute to a long civil service delay in responding and should not allow out a cop out answer or a refusal to help.
Some bad Ministers are simply out of their depth. They read out civil service responses without understanding what has been written down. I remember sometimes in Opposition asking a Minister to explain a complex or abstruse sentence they had just read out. A bad Minister would wait for a reformulation from officials. If I was cruel enough to ask for that to be explained I was promised a written reply at a later date as they did not understand the civil service reformulation either. Sometimes the point I was making was that the prose was deliberately abstruse, ambiguous or wrong.
I remember serving on a Statutory Instrument Committee where a Labour Minister put before us a short SI setting out the annual housing grant monies by Council which needed Parliamentary approval. The Department was late with the SI owing to incompetence. I noticed immediately that the SI recorded all the full amounts of grant being paid but also had on it the statement that all amounts in the text were in £ millions. The lazy Minister and all the officials had failed to spot that they were asking for approval for a total sum in excess of annual GDP! I proposed the Committee be abandoned and they came back another time with an accurate SI, as SI s cannot be amended in Committee under House rules. After anguished consultation the Labour Committee chairman and the government majority decided to just pass the wrong text!
A bad Minister either gets thrown out for bad conduct or leaves office with nothing positive to show for their period in office.