I think it is a fair question to ask when the president has the legal authority to ask for an investigation. But remember, he didn't ask the US Legal System to investigate, he asked a foreign power to investigate a political rival. It may come down to whether you asked in bad faith. However, I would argue that in most cases it is wrong when you're asking a foreign country to investigate. Like I said earlier if the legal system of the US had credible evidence and then asked a foreign country to cooperate in an investigation, that would be okay.
If you think this is okay, then I would ask you to put the shoe on the other foot. Would it be okay for a Democratic President to ask a foreign country to look into something that his / her probable chief political Republican rival "did". It should also be pointed out, that there is not credible evidence that Biden did anything wrong. For example, the company his son worked for was NOT being investigated at the time. It was in the interest of the US to get a corrupt prosecutor fired. Is it in the interest of the US for the Ukraine to pursue a discredited theory? Is it in Trump's interest? Should payments for the defence of the Ukraine already approved by Congress be withheld for Trump's personal benefit?