I agree with Starry Night. Everyone dissociates to some degree sometimes (for example while driving home from work or when you're not interested in what the teacher talks about in school). Spacing out is a form of dissociation.

If it does not interfere with your life in general, it is fine. For me, I can tell it's not fine when there's an emotional situation that triggers it and I can't get out of it anymore. I might sit there for 5 minutes while my T tries to get my attention and I don't notice he's even there. In those cases I'm generally in a state where if I were out on the streets I might walk in front of a car just because I was spacing out so much.

Whether or not it matters in my opinion mostly depends on whether this is a repeated pattern in certain situations. For example whenever your T asks about X you dissociate. For me that usually happens with things that were traumatizing for me. That's a good indication that on the one hand there's something there that might need some investigating and understanding. But on the other hand my brain doesn't want to go there right now and I should slow down a bit. In such cases it is helpful to notice this, since it can help you to not get overwhelmed. You can tell your T when you start noticing you dissociate and practice for that not to happen or what to do when it does happen.

But if you just were not engaged for some reason, or if it's a one time thing that doesn't happen again or is not bothering you, then dissociating is a pretty normal response.