Learning mussar is good for everyone. Period. If the question is whether it will alleviate the personality disorders of narcissistic or sociopathic personality, that is rather tough. The expectation I would have, as a few earlier comments pointed out, these people are unwilling and unmotivated to change anything. And, as one comment suggested, the person with these conditions does not lose their free will, and can change behavior.
RJ pointed out that “mussar does not work”. I must believe that this statement was poorly worded, and I suspect a few other sentences in that comment were similarly not well expressed. Mussar works great, and that is why so many works have been written on the subject. That is why reb Yisroel Salanter ZT”L made a bigger issue of it, and the many yeshivos that followed that derech pushed the agenda. I suspect that this has diminished greatly in efficacy because it is being approached incorrectly. Mussar is NOT an academic subject, and it is NOT an intellectual pursuit. It is a derech of how to guide one’s attitudes and behavior to become a paragon of kedusha, encompassing both realms of בין אדם למקום as well as בין אדם לחבירו. I also believe that those mussar works that do not make a greater issue in discussion of the regular חשבון הנפש are simply directing their attention to the content of that process, feeling there was little need to emphasize the obvious.
Basically, mussar, when approached as a practical matter, not simply academic, is a staple in Avodas Hashem, and benefits all who follow it.
Lastly, there is a quote from Harav Shalom Schwadron ZT”L wherein he stated that “Teshuvah is like a washing machine; it makes clothes clean, but it does not sew on buttons.”