If the person using the credit card is not the person whose name appears on the front and consequently the person who signed it, then you can't use it. I know there is a gray area where there is a joint credit card but then your mom would have her own card with her name on it. (same account number, different name and signature)
Do some people "get away with it"? I'm sure they do. A lot of people never check signatures. However, the retail company I work for part-time will not accept it unless signatures match. I know, I've had to be the bearer of bad news and hence on the receiving end of an irate customer or two. Really, it protects the consumer from fraudulent activity. That's what I tell the customer and I am backed up 100% by management.
What about getting her one of those Visa or Mastercard Gift Cards? It works the same way as a major credit card but has a finite spending limit.