I'd honestly say that Otto and Neckermann are offering catalogs more out of habit at this point. Your mom likes flipping through them - that's totally valid and if she enjoys it, there's nothing wrong with that. But when it comes to actually ordering, she simply has more advantages online: better prices, current availability, faster delivery, and the ability to check customer reviews right away. In the catalog, you never see whether an item is out of stock or if other people say the quality is terrible.
A practical compromise: She can keep ordering the catalogs (they don't cost much or are sometimes free), flip through them to relax, but when she actually wants to buy something, she does it online. That way she gets the best of both worlds. The online sites aren't complicated at all - just find the product from the catalog and enter it online. We did something similar at my place and my grandma thinks it's great. She loves the catalogs, but when she orders, she still uses the website because delivery is faster.
Bottom line: The subscription is only worth it if your mom really enjoys flipping through them. But for pure ordering, I'd definitely recommend she shop online. She'll often save money that way too.