Using a computer in the 80s is VERY different to using a smart phone or tablet in 2023. It was all keyboard and numeric menus, i.e. press 1 for data entry, press 5 for reports, press 9 for settings. Things changed massively in the 90s and noughties with "point and click" either with mouse or touchpad. You really can't compare a 80s PC with a modern smart phone or tablet, especially if your mother didn't move with the times or didn't regularly use a (more modern) PC in latter years.
My mother was the same. Extensive use of PC in office work in the 80s, but when she retired, she didn't have a home computer, and continued using an old fashioned mobile phone until a couple of years ago (keypad for text etc) rather than a touch screen. The phone broke as it was over 10 years old, and she got a new style iphone but has never got to grips with it because she just can't understand the idea of using a touchscreen. That's not because of her age (well it is as she can't learn anything new), but more because the way you make a phone call or send a text is completely different that she was used to.
It's like comparing the keyboard driven old Lotus 123 spreadsheet with the mouse/touchscreen driven Excel spreadsheet. Same thing, but very different user interface. My mother could probably still use Lotus 123, but she'd not know where to start with Excel. That's not her forgetting how to use a computer, it's the user interface moving on and leaving her behind because she's not kept up, and as time passes, the chasm gets bigger. The earlier versions of Excel
incorporated the backslash \ menu driven system to help people move over from backslash menu to mouse, but if you weren't actively transitioning and kept using Lotus 123 and then ultimately stop using it, you'd have a much harder job coming back and starting to use Excel 20 years later.
80s computers and original mobiles are nothing like modern ones. Unless you move with the times and replace old tech with new, then you're going to fall behind whatever your age. Sadly, a lot of older people saw no need to keep up and continued using old tech (or no tech at all) despite all the warning signs that literally everything was going more and more "techy" so have caused themselves to fall behind. Use it or lose it!