It is awful, what a cheek. I do not understand people even noticing who or where other drivers park. The person who left the note obviously has too much time on their hands. However, the woman did nothing wrong, she was collecting her child from Sainsbury's as well as her mum so was entitled to park there.
However the world is full of strange people minding the business of others; it hardly warrants a newspaper article but at the same time I hope the note writer reads it.
(Why she said, "So today I drive to..." and, "I go to...", rather than, "Today I drove.." and "I went to", is another matter.)