I guess I should probably expand on this just a bit.
If your taking portraits you pretty much have control over the lighting, the environment, etc - so really any camera that can capture a decent image will work fine.
The advantages to most mirror less systems is that they are generally smaller and lighter in weight. So if that's an important consideration, that might be a reason to consider mirrorless.
The advantage of the DSLR is that your going to have more options in the way of lenses and accessories, and if you decide to expand from just doing portraits, particularly into areas where telephoto or action shooting becomes more important, a DSLR might be a better choice in the long run.
But really when you have the time to set everything up and control all the variables such as lighting and location, etc - either will do the job just fine. You'd be hard pressed to tell a picture taken by a mirrorless vrs one taken by a DSLR. Even in environments you don't control, ultimately skill is a bigger factor in determining the end results than equipment. Equipment can just make certain things easier, that's all.