I have been to the zoo on Mother's day, and yes it is a bit crowded. Sundays have quite the local crowd normally, and add in the "special" day, it is a bit more.
However, the zoo is really large, so other than the entry plaza area, the zoo rarely seems too crowded to enjoy.
As a local, we do the 9-11:30 thing because as soon as the place smacks of getting busy, we flee. So, get there early and as it gets more crowded, take a deep breath, and shuffle with the crowd!!
Amy