This is actually quite a witty comeback/pun involving misuse of the phrasal verb 'zero in' in relation to the previously mentioned 'coupla tweaks away' (coupla = couple of = two) and 'work out this one thing'.
zero in (v) - (1) to aim directly at (a target) (2) to direct one's attention to; focus on; concentrate on (3) to converge on; close in on.
Zero in as mentioned in other answers is a phrasal verb most commonly used (in my opinion) as business language when trying to focus on finding some answer. Its literal root is to do with closing in on a physical target. Both of these points are important when considering the script.
The first important line is:
KRAMER: Well, you know, I really like this girl and I, you know, I think if I could just work out this one thing...
Here, Kramer mentions that he is trying to work something out although it isn't clear what that is from the given text. The next important line is:
ELAINE: (interrupting) Yeah. I gotta be honest with you Kramer. You might be more than just a coupla tweaks away from a healthy relationship.
So, in response he says:
KRAMER: Well you're not exactly zeroing in yourself, lady.
It's perhaps important to know that "you're not exactly 'blah' yourself, lady" is a common format of an insult.
So, now do you see the pun? Essentially, Kramer is trying to work out how to make his relationship work, Elaine responds by saying that he is a distance away from a healthy relationship. In his response he says that Elaine is not 'zeroing in' herself by which he means both that she is a distance away from a healthy relationship and he is referencing the ability to work out relationships.
Simple answer: When Kramer says Elaine is not 'zeroing in' he means to say that she doesn't have a perfect relationship either.
Complicated answer: On further inspection I believe there is much more going on in the script than other answers give credit. In addition to the simple answer, Kramer's response also references himself trying to work out his relationship and it continues the use of numbers throughout this exchange. I doubt this is an accident and reflects clever writing skills.