I don't see 'ok boomer' as a hostile attitude, but rather as a response specifically towards the media that has been calling millennials lazy and entitled for the past decade, despite the fact that education, housing, and healthcare and much more expensive for them than they were for boomers. It is dismissive, but I would say those who spout these generalized, ill-informed complaints have earned it.

And I'm sure there are some who use the phrase to refer to every boomer, but I don't believe most do.