So here is where I make my "In The Defense of Lacey Hamilton" case, which I realize probably isn't the most popular side to take.
Let's face it Lacey is a basically a child. She had Jenna young and had to "grow up" real fast, only problem being she never really did grow up. From what we've been told Lacey's relationship with her own mother was not the greatest. This is alluded to in the very last episode of season one, when she tells Jenna how she believed her mother loved a dress more than her. She spent her entire childhood trying to please a woman who just couldn't be pleased.
I can not imagine her mother was all too pleased by the fact that her daughter got knocked up in high school. Not to mention the fact that getting knocked up, probably didn't do wonders for her popularity back in high school, taking away from her the two things she desperately craved, affection and acceptance.
Cut to Lacey as we know her now, is she a little shallow? Yes. A little self-absorbed? Of course, but really when it comes to Jenna, despite not knowing boundaries and such, her heart is in the right place. She desperately wants Jenna to confide in her, she on more than one occasion reminds her that she isn't that much older that her, and she still remembers what it's like to be a teenager. She also tries on multiple occasions to connect with her daughter.
Sure, she's a bit misguided, more concerened about her daughter's popularity than Jenna's own wishes, but when it comes down to it she fiercely loves her daughter. Remember her freak out on Ally for slipping Jenna a Beta-blocker? Or the sweet reassurance she offers to Jenna, when she goes to talk to Tamara? As well as my personal favorite mother/daughter moment before the Winter Formal when Lacey tearfully tells Jenna, ""...you are the best part of me...because you're not me. You are better.""
The letter was definitely the wrong approach, but I have to believe that Lacey did it because she could think of no other way to reach her daughter. Now for all of you who are now furious with me and shouting varying versions of "That is NOT how a parent should act!" Please know while I agree with you, I ask you to realize that Lacey has never really needed to be an adult before. She had Jenna do young, and seems to always be trying to regain her youth. From what seen, Kevin seems to be the "stricter" of the two.
It's no secret that despite the fact that Jenna loves her, and she does, that she doesn't respect her mother. So Lacey had to believe, at least at the time, that writing the letter was the only way she could get through to Jenna. She was bound to pay more attention to advice, no matter how harshly put, from "a Friend" and peer than Lacey herself. So she did what she thought she had to to get Jenna to listen to her.