Well-rounded student does multiple sports, does science competitions and writes on their free time, and even cooks.
A student in a well-rounded class might be pretty good at debate & speech, participating in many government-related activities. They might be good at one sport, like soccer.
That doesn’t mean well-rounded students are frowned upon. The admissions committee looks at the class as a whole and fills in the gaps. If they need a trombone player and the well-rounded student is a good trombone player with 3 years of experience in honors band, they might take that student. Institutional priorities are out of your control, so like everyone says: JUST BE YOURSELF.