Whether it's "good" depends heavily on the context:
* Program: Some programs are more competitive than others. A 3.1 might be acceptable in a less competitive program, but it would likely be a detriment in a highly competitive one.
* Specific goals: If your goal is a highly competitive job or further academic pursuits, a 3.1 GPA could be a significant hurdle. If your goals are less demanding, it might be sufficient.
* Other factors: Strong research experience, publications, letters of recommendation, and other achievements can sometimes offset a lower GPA.
In short, while not disastrous, a 3.1 GPA in graduate school warrants careful consideration and likely requires some explanation or compensation with other strong qualifications.