You know, considering his voice objectively, I can see how it would not be everyone's "cup of tea". David Sylvian is definitely an acquired taste. I must admit the reason I bought my first Japan record was for "Visions of China"---I thought it was the ultimate in funk song I had heard at the time, and still love it. It was "my song" for a long while, but that is as far as the fascination went---other than "Ghosts" I had a bit of a time getting into that album, and pretty much thought David was the only one worth looking at.
Zoom ahead twenty some years, and I can hardly believe I was so lukewarm on them as I had been. Yeah, Tin Drum really grew on me, and from time to time I would drag it out for a listen every year or so, but now, heck, I can't imagine a day going by without listening to something of theirs or David's. Just the way things happen I guess.
I can say Morrisey's voice used to grind on my nerves when I first heard it, but I acquired a taste for his voice too.
Your Mum may never acquire her taste for David, but you never know---she very well could to your surprise!!!!
Heck, my Mum surprised me plenty growing up----she likes David Bowie, Boy George, Haircut 100---I was impressed. In turn I loved some of her favorites like Judy Garland.