SAHM here and I do put on makeup every day. It makes me feel and look more awake and human. I've never gone anywhere outside the house in pajamas. Yoga pants, yes.
But - you don't know what that mom had going on that day. Maybe she was having the morning from hell - sick kid, sick mom, etc. One thing motherhood has taught me is that you really, really don't know what other people are going through. Because I've had rough crappy days and realized no one else has any idea what is going on with me.
Btw - how do you know she was a SAHM?