It's ultimately the purview of the marketeer but they definitely should consider any input designer might have. Far too many marketeers want to be the designer and art direct their projects into hell. Most of the time I get to the point where I just say fuck it (to myself of course), "what do you want?", and just do that.

Well, what do we mean here when we say "banners are about" marketers or designers - or - designers/ marketers are "responsible" for banners?

The vagary in this set-up sounds like a source of the problem.

There should be a clear delineation of roles, responsibilities and deliverables when doing marketing design and collaborating with marketing stakeholders.

It sounds like the process is broken and designers have no say/ control in the process in some scenarios.

The first step to me is simple. It all starts with a brief that is written by a marketer, which is a subset/ tactic of a marketing strategy. Where does this banner deliverable fit into a campaign? What are the other contingent tasks? Rarely is a banner created that exists in lone isolation.

I don't know how many garbage marketing design briefs, say for banners, I've seen over the years. "Copy" that is a pile of "whimsy and clever" word soup and ill-respective of the limitations of the medium at hand. Image requests with no image acquisition budget. People playing arm-chair decorators with zero regard for brand and communication standards. No strategy for testing and scaling up or down the campaigns and so on.

Thus, you can continue to receive garbage briefs (if at all) or you can drive the process, educate and receive well done marketing design briefs and requirements that you can translate into designs.

If they can't write it clearly, how can they expect you to read tea leaves and "design it". So, get the requirements.

Finally, the actual feedback and iteration process is up to the designers to drive and educate marketing stakeholders in order to set them and everyone up for success. Create a "feedback process" - that is loyal to your brand and design system standards, which effectively neutralizes any deviations.

Hate to say it but I believe the marketing staff should be the one who is calling the shots when it comes to social media banners. I know it's important to give a certain freedom to the designer, but the marketing team, knowing the message that they'd like to convey better, should have the final word.