This is a hard question. First, I would try to find out why she isn't bathing. Keep in mind that some people have never bathed that often, and don't change this habit as they get older. Try asking her why she doesn't want to bathe. If you can get a conversation going, this may shed light on what's going on.
You should also ask your mom's health care provider for advice. A check-up is probably a good idea. You may also want to have your mom assessed by an occupational or physical therapist, who can help determine her ability to bathe, and suggest ways to make it easier.
Here are a few things to consider. Is it hard for her physically? I know people may look able-bodied, but this doesn't always mean they are. Perhaps bathing is painful; or it may be difficult for her to get in and out of the tub. Medical problems, such as asthma or heart failure, can cause shortness of breath during activities such as bathing. Perhaps bathing is too strenuous for her, and she's not sure why.
Also consider mental health issues. Many people with even mild depression or dementia resist bathing. People with dementia or Alzheimer's can be scared of water. They can also be annoyed by the feel or sound of water. If your mom is depressed, she may simply be too apathetic to bath. For people with mental health issues or dementia, bathing can be a very complex activity. It can be overwhelming.
Once you learn a little more, you can try helping strategies. Your mom may need personal help and direction (verbal cues) to bathe herself. If she's depressed, antidepressants can help. If bathing is too strenuous or painful, adaptive equipment such as shower chairs and grab bars can really help.