Honestly I don't think there is a character on Son's of Anarchy that you can truly like. They all have something they've done or helped cover up or otherwise done wrong. Wendy has her issues, but as a character I think she has an interesting trajectory and I think Drea de Matteo has done a great job bringing her to life.
While Wendy seems to want the best for Abel (and Thomas in later seasons), her repeated relapses with heroin clearly make her an unfit mother. She's proven time and again that she's unfit because she always goes back to the drugs. Even in season 6, when she almost had the kids, she turned to drugs once again and hit rock bottom.
This is something I feel I have zero right to really answer. I don't watch Son's of Anarchy. I don't watch much of anything on TV. Saying I disagree seems to be something that would equal out to a nice middle ground as opposed to agreeing and not knowing who Wendy is.
I feel that Wendy in Son's of Anarchy is not a likeable person for several different reasons. Wendy is cunning and manipulative. A former/current drug-addict, she has used several different measures to disrupt the lives of those around her. She has used extreme measures to try and gain custody of her son. Although she is an intriguing character, her actions have also made her an unlikeable one.