Is it still good for applications to be in a club where it isn’t competitive and can’t be a president/leader (no one can)? I’ll be in it regardless but should I build up other activities as well? It’s an academic club in school (math/science) that reflects what I want to major in
It’s definitely okay to be part of a club simply because you enjoy it, especially if it reflects your future interests. However, if your application is completely devoid of any leadership, that may raise some red flags to admissions officers.
Luckily, there are several solutions to this. You can, as you said, join some other activities, but you can also create some opportunities to lead in the club you are already in. For instance, if there are some people who are currently in the club or may want to join, but are struggling in a certain area, you could organize some of the members to form a tutoring group. Be creative, and do what works for you.
Being a leader has nothing to do with holding an office. In fact, I would say that in high school especially, holding an office has more to do with popularity than leadership. Be an active member of clubs and groups that interest you and you will most likely find yourself in a situation where you can display true leadership. Being the worker bee who coordinates volunteer tutors after school shows far more leadership than a president who does nothing.
Is that the only club you are in? Do you do anything outside of school? Sports, music, house of worship?
You don’t have to be a leader of a club to have leadership traits. These "leaders’’ of school clubs probably aren’t real leaders.
A title does not always equal leadership. The members of the club can demonstrate leadership in other ways. Someone suggested organizing tutoring- you could also host a bake sale to benefit the school in that area (I know my school’s STEM facilities are always underfunded, for example), you could host a 5K run to benefit a charity relating to this interest. Leadership means getting out there and making change, with or without a title.