I’ve committed to transfer to UIUC for psychology this fall from a community college. My #1 priority is getting a phd in clinical psych, probably focusing in severe mental disorders or eating disorders. UIUC’s current lab listings are geared more towards child development and cognition. Will doing research in an area of psych I don’t want to pursue in grad school hurt my chances? Also, how beneficial would it be to do independent research and write a bachelors thesis? Sorry for the loaded question but I legitimately will decline UIUC’s offer if this will be a negative factor, graduate school is hard enough to get into 
So first of all, UIUC has one of the top clinical psych programs in the country (and just psychology departments, period) and I know for sure that there are several professors there who do research on severe psychopathology/mental health disorders. Just by my count in the department, there are six professors doing research on psychopathology (Howard Berenbaum, Joseph Cohen, Benjamin Hankin, Wendy Heller, Thomas Kwapil, Heidemarie Laurent) and three more who are doing work that has easy intersection with psychopathology (Catharine Fairbairn, Carla Hunter, Nicole Allen). And that was just peeking at people in the clinical/community area; psychologists outside of clinical psych do research on psychopathology all the time. So there look to be plenty of opportunities to get into research in psychopathology.
The other thing to remember is that you’ll narrow down your interests and you’ll look for interesting cross-sections and interdisciplinary foci. So severe mental health disorders…what about them? Severe mental health disorders and…adolescents in the juvenile justice system? And racial health disparities? And their impact on parenting and child development? And…and…and? By doing research with a professor that’s maybe slightly outside your main focus area, you can uncover interesting focus areas that no one is doing and that may even make you more interesting to a grad program.
With that said, no, doing research in an area that you’re not planning to pursue won’t hurt your chances. It’s nice to get close, but the important part is developing research skills - basic psychological research skills span subfield and particular area of interest. You don’t have to get super focused until you are looking for doctoral programs.
Doing independent research and writing a thesis is very beneficial! Not only does it look great on applications, but it helps prepare you for the kind of independent work that you will do in graduate school as well.
You’re right I really haven’t narrowed down my interests all that much yet… hopefully that will come with time as I begin studying at a 4-year. Thanks for replying this was really helpful and I’m not as nervous about the whole lab situation anymore 
It’s good that you haven’t narrowed your interests yet! You shouldn’t have! It’s still early days. My point is simply that since you haven’t this is a good time to explore what kinds of research is out there and get immersed in the scientific ‘conversation’ currently happening within different subfields of psychology around mental health disorders. Your experiences will help you figure out what you’re interested in, but nobody is expecting you to have it all figured out yet
Good luck!