At my employer, engineers at every level are accountable for their work's business impact more than anything else. It is always your responsibility to understand and be convinced of the value of what you're doing, and to escalate your concerns or transfer out of the team if you're not. "The product requirements were ill-conceived but I executed them well" will not save you at performance review time, even if you're a new grad.

A competent PM has considered these questions carefully, and most of their thinking should be written down and circulated before the project starts. They will happily discuss lingering questions with anyone interested.