The accents are just one of the many examples of how poorly and without attention to or care for detail Thomas Kail's direction was.
I agree the accents of most of the cast are pretty bad. Where was the damn dialect coach? Hire another one, and another. And how bout casting people who can DO a British accent. In so many cases it seems like the ability to do a solid accent *before* starting rehearsals is just no longer a prerequisite for being cast in a Broadway production.
Some very talented people just don't take well to accents. That's not their fault, and I hope they have a thriving career doing American roles. But they shouldn't be cast in major productions that require accents.