Can the debtor asks to defer wage garnishment for a few months?

Usually no. However, a portion of the debtor's wage should be garnished based on the type of the debt he/she owes.

What type of debt are you talking about?

As per Federal Law, usually wages can't be garnished more that 25% of your disposable earning. That means, the maximum wage that can be garnished if your 1 week salary. However, the wage garnishment depends on the type of the debt. You can contact your State labor department to know about the wage garnishment laws in your state.
However, you can file a bankruptcy to stop wage garnishment.

Why do you want to postpone wage garnishment? Have you lost your job? Did you get a pay cut?

You can challenge a wage garnishment order within 30 days to delay the garnishment. But you need to have valid grounds for your objection. Otherwise, it will be futile.

If you have a regular income from your job, you will not be able to stop or postpone the garnishment. Filing for bankruptcy protection will be your only option. And, your ability to file for bankruptcy is to tally depending on whether you are eligible to file in the first place.

You can apply for a hardship hearing and if everything happens perfectly, the garnishment might be reduced to maximum 15%.