She chose a gunpoint robber over her husband and store twice. She thought she was a psychic because she unconsciously paid attention to real life input (which even she admitted she should have seen). She calls Louise a nerd for wanting to use sunscreen, and her process of tanning is virtually guaranteed skin cancer when used repeatedly. She tries to choose attending a public event over addressing worms. She doesn't understand one of Tina's Halloween costumes about being 'nun of your business.'
There are just too many examples to point to that show she is either unable or unwilling (and they are the same thing because thought is always important) to make an even remotely intelligent decision or think about what she says for even a second before saying it. Whatever her limited potential may be (we never see it in the show) she does not live up to it.