I cannot be sure about my Dad but my Mum,despite her relative lack of "formal" education,routinely completed the Sunday New York Times crossword puzzle.
In pen.
My guess is that she would have held her own in any Scrabble game...at any level.
I am sure your mum was a very astute and talented person, but that only testifies to your well-placed filial loyalty and limited sample size.
Anyway, when I was a young teen, my Dad told me that he would give me $5 if I could do the NYT crossword puzzle. I took the challenge. I cut the puzzle out out of the paper and went at it. In about an hour, I presented the result to my Dad. He was amazed and duly impressed and even a bit proud that I was able to do it. He then handed me $5 for doing yesterday's NYT crossword puzzle. I confessed 40 years later.