Is the word 懵逼 dirty?
It's not polite or appropriate, but has been used so much by teenagers on the internet that it has become quite acceptable on social media. Middle-aged people will consider it a very bad word to say. 

A more appropriate substitution for it could be "懵圈". However, both words should never be used in an official context.

And yes, 逼 is a substitute for 屄 and is more commonly used.(Both characters I've rarely used before answering the question...)