This is one of those things that may vary a bit from state to state. The prevailing theme in most states from what I can gather is that courts don't put much stock in a label unless you can directly tie the disorder to the best interests of the kids. This is best done by associating an already-observed pattern of behavior with a negative impact that has already occured on the kids.
However, you really need to dig into your state's code on this one. For example, in my state, GA state code 19-9-3(a)(3) (as recently modified and effective 1 Jan 2008)
(see page 4) outlines 17 factors or guidelines that a judge may
use to help him determine the childs best interests. Factor I reads: "The mental and physical health of each parent." So in GA, a diagnosis of mental health is legislated as a legitimate discriminator that a judge may
use during his decision making process. Keep in mind that it is only one factor out of 17. Going back to the original point, if you can attribute behaviors to impacts on the children, then you can use that to influence some of the other 16 factors. I think in your case you might be able to do just that.
That said, state code generally doesn't mandate what factors a GAL or CE would use in coming up with their recommendation, and even if they do follow the guidelines, there's no guidance as to how to weight the factors against each other. So a GAL or CE could at their discretion put a great amount or little to no weight behind such a diagnosis. I think in most cases, how a judge weighs these factors will probably be consistent with how the GAL or CE weighs them, as judges won't often trump these parties.
Also to consider is that judges are wildcards. They have to follow the law, but the law in this area affords them a great amount of discretion in these cases. That said, a judge's individual biases and past experiences with personality disorders come into play. The label that comes with a psychiatric diagnosis may significantly impact one judges decision making process, or it may have no weight at all until you tie it to the impacts on the children (especially in cases where there is no CE or GAL).
Consequently, it's hard, if not impossible, to give a blanket answer to that question that is universally right. In your specific case DMK, it's clear that your judge is biased and is abusing her discretion, and you're going to have to appeal her at some point in the future to get a fair shake. So, the question you should be asking is how best to frame your case for an appeal? That drives another question...what angle are you going to use to attack the judge, misapplication of the law, or abuse of discretion?
If you go for misapplication of the law, then you need to see if PA has a code similar to GA, and see if your state has legislated factors for determining custody. Then, you need to see how the specific language is worded...does it say the judge MUST consider those factors, or the judge MAY consider those factors? If the language is MAY, then building a case around this diagnosis with an intent on appealing on misapplication of the law is probably fruitless. If you're going for abuse of discretion, then you need to look at those factors (if they exist in your state's code), and build your case so you get professional third parties when possible saying as many of those factors are in your favor as possible, which will make framing an appeal on abuse of discretion easier to build and argue. In that case, I would make sure to introduce as much as you can, including a diagnosis, but I wouldn't spend too much time on any one argument as if it were a silver bullet in determining your case. Of course, consult with your attorney first on this part of your strategy before moving forward. Just make sure your attorney's strategy is clearly defined as to whether you are more interested in trying to win this round (if you think you have a chance with your judge), or if you are going to frame it for an appeal, and if so, on what grounds are you going to appeal.