Lead quality is a plumbing problem
The conversation about lead quality usually starts as a disagreement. Marketing points at cost per lead and volume, sales says the leads are useless, and both are working from different data. The way out is to make quality a measured thing rather than an opinion: every lead gets a status, every disqualification gets a reason, and cost per qualified lead is reported by campaign.
Once that exists, the diagnosis is usually straightforward, because bad leads are rarely spread evenly. They cluster in particular search terms, particular locations, particular hours or particular ad groups, and clusters are easy to act on.