If an employer is looking to hiring ASAP and you get the job offer, would it be a bad to ask for time to think about the offer?

Just ask what the deadline for your response is. Any reasonable employer will give you 24-48 hours to respond.

do your thinking about accepting this job before you apply, once you apply you are expected to be ready to take the job if they offer it to you since they need someone asap

You are absolutely entitled to ask for time to think it over. I hope you also made a counter offer on the salary they offered.

If I needed to hire ASAP and you ask for even an hour to "think about it" then I move on to the second candidate.

"Could I consider this overnight & get back to you with my decision by noon tomorrow? You don't ask for 48 hours anyway. 24 is typically the max. If you know they're in a hurry, be willing to be a bit flexible. And do indicate that if you accept their offer, you will be able to start "immediately" or "Monday morning."

if you tell them that you need time to think about it, they will hire another person

Depends on how quickly they need someone..

You applied, which means you wanted the job. It makes no sense to 'think about it' at this point and that's the angle they're going to be coming at from their side.

You don't need time, you have been interviewed so you make up your mind when you leave the building.

They certainly won't wait for you dallying around.

No, it is not bad to ask for a day or two to consider the offer. If they want you to make a snap decision about a major life event like changing jobs, it's not somewhere you want to work.