It represents several different things. First of all, it has a sexual reference--implying her prowess in the bedroom. Considering Taylor has done everything in her power to not get pregnant and stuck in her town, this is highly ironic.
It also indicates the relationship that she will eventually have with this child. The "good" part of the phrase could imply a "good for you" idea. It is also interesting that of all of the t-shirts Taylor chooses to dress the baby in, she picks one that she highly treasures and that she feels good about herself in. That implys that that on some level she also treasures this baby as well.
It is the slogan on one of Tailor's favorite shirts and it signifies her sense of self which identifies her personal strength throughout the novel. At one point she wants to hand the shirt down to Turtle who needs to build her self-esteem.