When we first meet Walter White and his wife Skyler, she seems like she's the motivation for Walter to start making meth. After finding out he's dying of cancer, he constantly says he wants to make enough money so his family is financially secure after he passes away. However, as the show progresses, we learn that Walter's motivation may be slightly more selfish than he originally stated; years earlier, he founded a company with a former girlfriend named Gretchen and a classmate/friend. Walter sold his share of the business after a falling out, it became incredibly successful after he left, and Gretchen married his friend/business partner. Walter took a job as a high school teacher—one he was overqualified and underpaid for—and married Skyler.
To give a greater sense that Walt missed out on more than just money and a successful career, Skyler's character needed to be somewhat unlikable. This would contrast her with Gretchen, the one who got away, and contribute to Walt's belief that he had to settle for everything in life, even the woman he married.
Another problem that Skyler had was that she was tasked with being the voice of reason in the show. While the viewer wants Walt to continue with the meth business and do well, that's not realistic. If your partner, friend, or family member told you that they were going to start making and selling meth, you'd lose your mind—it's one of the most toxic and addictive drugs available today and users, as the show clearly demonstrates, can be violent and unpredictable. No right-minded person would keep his or her children around a person who chose to make a living that way.
While Skyler played an important role in the show, it's too bad they didn't give her more complexity instead of settling for making her an irritating plot device.