To overemphasize something is to give it more importance than it deserves or than is suitable — the verb that sits beside the noun overemphasis. Teachers, managers, and writers use it a lot: Don’t overemphasize grammar and forget speaking; the report overemphasizes one bad month. It is a little formal, but not rare. British spelling is often overemphasise. Though actually, people also say put too much emphasis on, which is gentler and more common in speech. The word carries a quiet criticism: you are not just emphasising; you have gone past the useful point.