Garnishments are treated like any other debts for tax purposes. If the debt that the garnishment is collecting is deductible, you can use it to reduce your tax burden. It's the nature of the debt that counts -- not the manner of its collection. However, any wages garnished from your paycheck still have to be considered when determining your tax burden.
Same as Other Bills
The bills being collected via garnishment can be deducted only if they would be deductible regardless of how they were paid. For example, if you have severe medical bills that total more than 10 percent of your adjusted gross income, any bills paid as the result of a wage garnishment can be counted towards that deduction. In this scenario, you're still paying the medical bills -- the fact that you're doing so involuntarily doesn't negate that fact at tax time
Garnished Wages Count
If your wages were garnished, you'll take home less money in your paycheck. However, you'll still pay taxes on the garnished amount as though you were being paid your normal wages. That money is considered to be earned income and will be displayed as such on the tax forms your employer sends you at the end of the year.