Basically, the late Emmeline Grangerford's art and poetry was all completely morbid and sad. As Huck says about her poetry,
she could write about anything you choose to give her to write about just so it was sadful.
All of her pictures are of dead or dying things. She has one of a woman mourning at a grave. She has one of a woman mourning a dead bird. The one she was working on when she died was a woman about to commit suicide. Her poetry was also about dead people.
Huck says that he thinks all of her work is nice but that he can't "take to it. It is all too sad and it depresses him to see any of it.
You can find more details in Chapter 17.