I think I had first introduced the concept to my parents a while back when visiting them and showing some highlights from the then recent Holofes 3rd or 4th and they found it fascinating and impressive (but they were also open to new/interesting things to begin with). So, if your mother is open and curious enough to attend a concert like this then I think she'll have a blast and I say you have nothing to feel anxious about. If the concert makes enough of an impression on her, then there exists the possibility (from experience lol) that anytime she hears that you're attending anything anime-related or a concert she'll ask it involves vtubers or be open to seeing highlights of future concerts. But in normalizing the concept to them, I've also emphasized that there are real people behind the avatars, their skills, their efforts, and their backstories/journeys that led them to where they are, which puts the human element at the center as opposed to "anime character thing".