Using Credit Karma to check your credit scores will do no harm to your credit scores.
Even checking your credit scores through other credit score checking websites will not harm your credit.
The only time a ding gets put on your credit score when your credit score is checked is when a company checks your credit history.
But if you're just checking your credit yourself then your credit score will not be harmed.
I find credit karma to be the easiest website to use when checking my credit scores even though I use other credit score checking websites to make sure the credit score is accurate among all the credit score checking websites.
You can and should check your credit score as often as you can to make sure there's nothing wrong with it before going to get a loan or other credit.
Knowing your credit score yourself is a good thing before you try to apply for loans, auto loans, credit cards, mortgage etc.