I had a therapist when I was about 17 who seemed to doze off in sessions, but I was young, she was quite intimidating to me, and I found it hard to trust my perceptions. I brought it up once and she said she was just deep in thought, which still rings false after 20 years, lol.

If your therapist has a habit of yawning, it seems like a completely valid concern to me that she might not be attentive- does she sound present otherwise, and react to you as if she's well-attuned?

Personally, my therapist just yawned once, for like the first time about a week ago, and she excused herself as if she was embarrassed. I knew she'd not felt well the day before, she'd let me know she was a bit tired, and I offered to reschedule, but she said no need- so... in that case I wasn't bothered a bit, but if she did that regularly, I would, honestly, take it personally, that she was bored with me or unconcerned about my therapy.

I was basically raised to treat yawning like burping.... not something to give in to in public, and to excuse myself for if I did it inadvertently.