hugs, I'm so sorry about your mom, I hope that she is ok.
She probably wasn't sure if calling would disturb you or if it might take away from your time. Many people aren't comfortable with illness and don't have the emotional resources to deal with it. Frankly, they just don't know what to do.
Now, before you say, 'I would think that she should . . ' remember that you and your MIL are two separate people, you have different personalities, different life experiences, and different learning styles. What this means is that just because you wanted one style of communication, does not mean (not necessarily) that she intended an insult.
If you want to find a way to be mad or offended about it, you can do that. But, here is the thing: why bother? Would being mad about it help you? Would it make things better for you? Or does it just mask your own sadness and worry about your mother, and only provides an outlet for all of your feelings of helplessness? I know this sounds harsh on my part, but I have done a tremendous amount of hospice work in my life, and I see this behavior manifest itself all the time.
My experience has taught me that trying to displace our own grieving feelings (because a stroke is very scary no matter the outcome.) by delving into anger doesn't make the sad feelings less, it just postpones the acceptance of the fact that it is scary and sad when our parents are ill.
I guess what I'm trying to say is, yeah, maybe she could have communicated better, but it won't help you to spin your wheels about her social ineptitude.
Hugs!!