Isn't it kinda weird?

Yep. In fact, this pertanyaan has been asked before. The myth was that it was because back when Walt was alive, he made motherless characters due to the fact that he blamed himself for his own mother's death (she died of a malfunction in the house he bought for his parents). So people say that disney just continues the tradition.

However, it is most likely because having a dead mother invokes lebih sympathy than a dead father. Many people don't know their fathers at all, so a dead mother would make them feel sorrier for the character.

And BTW, Atlantis isn't a disney Princess movie (I know Kida's a princess, but much to my chagrin, she's not an official one).

From what I know is that most of the Princesses are from Fairy tales (with exception of Pocahontas).