Is it easier to get an Australian green card if you are single?

Of course there will be no impact.

Whether you can get an Australian PR mainly depends on whether you meet the PR requirements. It has nothing to do with being single and married.

After your studies are completed, and your age score + degree + IELTS score + other bonus points have reached an immigration score of 120, you can obtain a permanent residence visa.

As for your girlfriend, you can get your permanent residence first, then notarize your marriage certificate, and then apply for her direct spouse visa.