Marti has a ton of problems, and her mom is definitely one of them. For some reason that's beyond us to try to explain, Marti's genius dad who got his Ph.D. by the time he was 23 started going out with Rose Lee when she was "a sixteen-year-old high school drop out waitress with big boobs" (9.101). Now, she's basically a clone of Karl's mom, and the two of them are going out drinking together. Oh, good.
What's more, though, it seems like Rose Lee also has the same basic problem Beth does—never having established her own identity early in life. "I've been living so much in a man's shadow all my life," she tells Karl. "I got married when I was just a girl, and to see someone who doesn't let anyone put her down or tell her anything, that's just amazing to me" (21.50). Having gotten pregnant when she was still a teenager, Rose Lee seems to have spontaneously combusted as an adult in a failing marriage, identifying with Beth's tendency to drink away her problems.
The other thing about this lady is that she seems totally uninterested in her daughter. When Karl tells her how much he likes hanging out with Marti, her only response is, "'You're very kind' […] in that voice adults have that means thank you for lying, so nice of you to spare me the truth" (21.47). Um … has she met Marti? Marti's awesome.
Then, just a couple of days later, she's suddenly Mother of the Year by forcing Karl to get tested for STDs and verbally attacking him for allegedly sexually assaulting Marti. Apparently, she's only interested in Marti when it's convenient. The rest of the time, she sees her largely as a failure, just like her husband does.