Personally, now that all of my bills are paid online, I haven't had a useful piece of postal mail in years, unless it was a holiday/birthday card. So I would start by getting as much of her monthly bill payments as possible automated and handled digitally.
A lot of the rest, you can kind of tell by feel. If it's a catalog, or a coated envelope, or a single sheet of large card-stock or coated card-stock, it's 95% likely to be junk mail that can be tossed.
To reduce junk mail, she can opt out of things like pre-screened credit card/insurance offers:
$1And she can opt out of may other types of junk mail here:
$1If she's uncertain, she can use something like Seeing AI, or a magnifying app. A phone/digital magnifier may be easier to see than using an actual magnifying glass. For Android, I like WeZoom (free) - lots of options for controlling contrast and color modes.
If there are bills/regular payments you can't automate, or she goes somewhere where the knows they'll be sending her important mail/bills (like a visit to the doctor), ask if she can pay by phone. For monthly bills, she can call them each month and pay. For other things, the respective billing department for whomever she's dealing with can call her. If paying by phone isn't an option, she can request to have those things sent in large print.