To start off, she is willing to take Huck in and try to take care of him: "She took me as her son, and allowed she would sivilize me." He is a pretty rough-around-the edges kid, with little manners and not much upbringing. Her willingness to take him in shows just how good of a person that she is. She also shows a lot of love and concern for him. On his occasional run-aways, she is very worried, and "cried over me and called me a poor lost lamb". She also tries to teach him manners, and about the bible, praying, and heaven. So, she's doing her best with him. Then, when Pap comes back and is trying to get Huck's money,
"the widow went to the law to get the courts to...let [her] be my guardian."
So she not only takes him in, but tries to save him from his abusive father by trying to adopt him. She takes action to keep him for hers for good. This reflects her parental nature, good sense, and kind heart.
Her parenting is a bit naive though; she takes a pretty uncivilized Huck and expects him to be mannered and still right away, and to get everything perfectly the first time. She was also a bit of a hypocrite, telling Huck to stop smoking because "it was a mean practice and wasn't clean," but at the same time, "she took snuff"; so, she was giving out orders that she wasn't keeping herself.
Other than a bit of naivety and being a bit hypocritical on some things, I would say the widow is doing a good job, doing the best that she can, given her circumstances. Huck is lucky to have her.