That's a good point and I think people can argue for and against it, like for neutrality or showing emotions. A lot of times when people have shown strong emotions, that's when I felt they actually cared about an issue. I had a therapist was so neutral about everything I thought she would react the exact same way if she heard her child died. In the whole year I saw her she never showed any strong emotions. I never saw her teeth, she only smiled closed-mouth a few times but it seemed forced. Never anger, never surprise. Like those people who had a lot of botox done.
But I can also see the downside of showing a lot of emotions, as others have pointed out.

I think the most important part is that you did not feel she reacted to you the way you wanted or expected. I feel like I say this a lot on this forum, but once again, why not bring it up with her. Ask your T, often the conversation that follows can give one better understanding.