Not all therapists engage in mirroring. It is a technique that can sometimes be used depending on the style of treatment and also how the therapist was trained. Usually when therapists engage in mirroring it's because they are connecting with you and reacting in a way to help keep that connection. Matching tone, posture, or body language is a way to do that. It isn't always effective or welcome to some clients. If you're noticing it, it's okay to comment on that too. Therapy can also be about the relationship you develop with your provider and the way she/he responds to you or supports you. If you're appreciative of the connection and the mirroring, it's also okay to comment on that.

In general I'd say that every therapist is a bit different and not all of them engage in this behavior. Some are more cold and withdrawn. My current therapist is definitely more objective and distant. She doesn't really emote a lot and she usually sits in a standard posture in her chair despite what I'm sharing in session. I appreciate that she has a great memory and is able to help me connect dots and offer perspective. I care less about her tone and body language and more about the content of what she offers. She's not perfect, but I find that she's effective for me. Fit is super important.