I would guess that your mom is correct and the balloon is semi-permeable to water. All balloons are not made up of the same material, however, so I could imagine that some would balance out faster than others. This is analogous to the case of helium-filled balloons, to which I am more familiar.

As you probably know, helium-filled balloons typically deflate within a day or so through normal balloons (latex). This is because the helium can pass relatively easily through the semi-permeable material. However, you may have noticed those silvery balloons that look like they're make up of some kind of foil. Those materials are typically much less permeable (if at all) which is why they can stay inflated so much longer than a typical latex balloon. Hope this helps.