If you would like to become a biomedical engineer, then definitely go for it! I don't know what ACT score you need, this is probably dependent on the university/college you apply to. I did my undergraduate degree in biological engineering at a university in Canada and we did not need to have ACT scores. That being said, I did need to do well in high school (especially in science and math) to have a high enough average to meet the requirements to get into the biological engineering program. Have you looked on the websites of some of the universities you would like to apply to, to see what ACT scores you need?
Additionally, I think that universities/colleges are looking for well-rounded students, not just students who sit in their rooms day and night studying. After all, wouldn't you rather work with individuals who have outside interests that they can draw from to help solve problems, than just what they learned in a textbook? Being well-rounded (i.e. participating in high school clubs, sports, volunteering in the community) shows that you can manage your time, be a leader and a team player, and take initiative, all important qualities no matter if you decide to be an engineering student, a psychology student, a biology student, a music student etc.
As for whether you need to be extremely SMART to be a biomedical engineer, I would say NO. You need to be able to have self-dicipline to do your homework and assignments, even if you don't particularly enjoy the course, you need to not be afraid to ask questions and get help if you are having a tough time understanding something taught in class, and you will need to make sacrifices sometime (i.e. you will need to spend a lot of evenings and weekends doing homework, group projects, studying for tests and exams). It's not a matter of being naturally super smart, it's more about applying yourself and doing the best you can, and not giving up if you can't figure something out the first, second, third etc. time. So, if you are willing to put in the effort, and you have a strong interest in biology, medicine, and problem-solving, then apply to colleges for biomedical engineering! You have nothing to lose, and might later regret it if you don't apply.
Hope this helps! Best of luck.