I have a fairly common surname and get a bunch of stuff meant for other people. My suspicion is that their legitimate username is just one or two extra characters after mine, e.g. "[email protected]" vs "[email protected]". People probably think that, as long as they are careful about communicating the right username, it will be fine; but communication requires skills on both sides, and it's easy to miss a single letter or digit in common conversation or in poor lighting.

I get occasional emails meant for my mom as we have similar email addresses. It's a common mistake made by many of her contacts, what is she supposed to do about it?

Use an email account attached to her own domain so any address at that domain goes to her.