Sure. Australia has had mandatory helmets since 1990, and there's been endless studies and debates since then, it's still ongoing. I could find no clear evidence that helmet mandates decreased overall harm over any timeframe.

To quote a review I read from 2007

The following general principles should have widespread support: (1) Any legislation (including helmet laws) should not be enacted unless the benefits can be shown to exceed the costs. Ideally, the benefits should be greater than from equivalent ways of spending similar amounts of money on other road safety initiatives.

And their conclusion did not find a consensus other than

A majority of brain injuries >AIS2 are caused by bike/motor vehicle collisions. Traffic calming, enforcement of drink-driving laws, cyclist and driver education, or other measures to reduce the frequency and severity of bike/motor vehicle collisions, may therefore represent more cost-effective ways of reducing serious head injuries to cyclists than helmet laws. Indeed, countries with the lowest fatality rates per cycle-km also have the lowest helmet wearing rates

Given that, helmet mandates are a bad law that takes away our liberties for no proven benefit.