There are a few avenues for exploration with your Mother. Firstly did she start any new medication directly after surgery? if so, have a look at the side effects for that medication. Secondly, traumatic surgery such as a bypass can really affect people in different ways. Some have been noticed to be depressed, some simply withdraw into themselves and become quiet, and others present no changes at all. The effects of surgery are different for everyone. I remember after my triple bypass I was very difficult to get along with. I just wanted peace and quiet all the time so I could handle the discomfort (which lasted a year) and concentrate on recovering. It was as though my body was zapping all my energy and there wasn't much left for my mind. A year after surgery my discomforts disappeared and it was like I suddenly had more energy and woke up. I didn't talk to my family about my discomforts and how I felt because I knew if you haven't experienced it, then it's a waste of time. Maybe your mother is feeling similar.
Thirdly, probably irrelevant, but still worth checking, is the possibility of a mild stroke. My Mother suffered one of those and she seemed to change into another person. However, after a couple of years, most of her came back. Heart surgery does pose a risk for strokes and although probably wrong, it may be worth asking the Dr what he/she thinks. Mini/mild strokes do not usually show the symptoms of the face dropping on one side or the body being weak on one side. Many people have mini strokes and don't even realise. When it's your Mother, you want to check all avenues, Mums are so special aren't they.