This is always an interesting topic - even a bit of a debate, maybe! There is no right or wrong answer to the question, to be honest.
I tend to fall on the side of "you should choose a niche and stick to it." For me, I think it's easier to be memorable and known for something specific. And I think audience members - whether they be buyers, readers, pinners, etc - tend to like to follow and consume content from brands / creators who are clear about something specific they help with.
Plus I also think it's easier to maximize SEO (search engine optimization) on Pinterest (or other search-based platforms) when you're clear on a few content pillars that you want to be discovered for. It's my belief that having content in many different "niches" or verticals makes it harder for Pinterest to understand what your account and pins are about. And as a result, your audience might not be as targeted.
BUTTTT 🙂 - I know there are varying opinions on this. AND I also know lots of creators (Pioneer Laura from @everydaylifewithlaura comes to mind!) who create in many different niches or areas - and do REALLY amazingly on the platform.
So, bottom line I think is that there's no right or wrong here. It sort of comes down to what you want your content strategy and your monetization strategy to be.
This may not provide you the exact clarity you came for - but hopefully it gives you some additional insights to help you evaluate your strategy!