This comment from Tom in Feb 16 suggests Monzo will not be mad crazy strict on this (afterall, they have no need to channel sole traders into a business banking offer):
"We don’t plan to offer full business accounts because the requirements are so dramatically different. Multi-user access controls, card acquiring, accounting, invoicing, etc, etc
Sole-traders and freelancers are probably close enough to a personal current account that Mondo would work for those cases."
So it seems depends what you are looking for. If you need the integration into accounts etc, then look elsewhere (there are posts that can help on this). But if you just need to keep eg your freelancing transactions separate from personal ones, then it looks like Monzo would be happy if you are happy to use Monzo.