Why does Meg say that she is a moron?

There are two instances in which Meg calls herself a moron. Both examples are included below.

"We know you have a great mind and all. Mother," Sandy said, "but you don't have much sense. And certainly Meg and Charles don't."
"I know. We're morons." Meg was bitter.

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"Now, you see," Mrs. Whatsit said, "he would be there, without that long trip. That is how we travel."
Charles Wallace accepted the explanation serenely. Even Calvin did not seem perturbed.

"Oh, dear," Meg sighed. "I guess I am a moron. I just don't get it."
"That is because you think of space only in three dimensions," Mrs. Whatsit told her. "We travel in the
fifth dimension. This is something you can understand, Meg.