I actually think she was a very driven individual, who was very dedicated to her duties in the castle. She was probably in charge of some important task, and thus was very serious. I imagine she loved Goofy because he was so easy going, and he was one of the few people who could get her to really kick back and forget about work.

Of course, she probably wanted to strangle him sometimes, and got exasperated at his clumsiness, simpleness, and tendency to nap a lot, but then his sweetness won her over every time. I also bet she was a bit sarcastic.

The reason I feel this way is it makes sense to me that Max actually takes after his mother a lot, which is why he shares a lot of her frustrations with his father, although like her, he can't help but love his dad at the end of the day.