How did you establish it was a "mom" (as opposed to an aunt or someone taking care of someone's kids), that she was texting with "her friends" or even texting at all, and what the topic of her texting was (if it even was texting)?

Maybe it was a girl borrowing her brother's (who is a dad: I am assuming you noticed child seats or kids) car looking at the weather forecast at her phone driving recklessly?

Yeah, I get annoyed at people using their phones while driving (both when they drive recklessly and when they drive too slow making everyone around them nervous). So you should too. Moms running errands should not be your target of anger :)