I'm not clear on what you were hired to do and it sounds like you weren't either. Sure, they said they want someone who isn't afraid to give feedback and improve the product. But it seems you took that one statement as an encouragement to run circles around the org-chart.

Were you hired as a programmer or a designer? If the marketing copy or the UI/UX is already clearly specified, as a developer you should be thankful. It makes your job easier when you aren't expected to 'improve' loosely specified things that aren't your specialty.

I'm a programmer in a three-person startup (along with the founder and a designer) and role overlap must be our biggest problem. The founder is the domain expert and the designer is an excellent graphic designer, and none of us can resist offering his/her two cents on the UI/UX - the back and forth is endless. As a programmer I'm lucky to not worry about the rest of the team reading my code and offering constructive criticism or insisting on a refactor.