There are only four characters in Tennessee Williams's The Glass Menagerie--five including the unseen presence of the missing husband/father. Each of them is more complex than they might appear at first, and that is certainly true of Amanda. The advantage readers/viewers have with her is that she is voluble (she talks a lot) and various aspects of her personality are seen throughout the play. Based on the things she says, her behaviors, and how others see her, she seems to be a more sympathetic character than someone the playwright condemns.
What Amanda says is annoying and sometimes ridiculous. She nags poor Tom about how he sits, about how he chews his food, about bringing home a suitable suitor for Laura, about not having enough ambition, about not making enough money, about going to the movies...and probably even more. This would make anyone as crazy as it makes Tom; however, most of these things are rooted in fear. She is afraid Tom may leave, she is afraid Tom will never amount to much, and she is afraid she will be left without a way to care for her daughter. While we may not admire her for this, we certainly should not condemn her for worrying about how to care for her family. Her speech after she discovers Laura has not been going to school reveals her fears about her daughter's future, something which makes her more likeable because we understand her motivation.
Amanda is an outrageous character in nearly every way, overly dramatic and larger than life; however, she works two jobs, pays for her daughter to go to secretarial school, and desperately wants to ensure her daughter's secure future. These are admirable things, even if she is rather a bulldozer as she does them.
The only character not related to Amanda is Jim, the Gentleman Caller. He has every reason to dislike Amanda--even be appalled at her outrageousness; instead, he likes her and appreciates what she is trying to do. He keeps a straight face when she appears in a dress she wore as a girl thirty years before, and he is charmed by her southern airs.
Given the evidence of her words, her behaviors, and the reactions others have to her, it seems as if Williams dis intend for her to be a sympathetic character at the least, and perhaps even an admirable one.