Is the word 懵逼 dirty? 懵逼 is easily construed as dirty with it's 逼 suffix. I cannot, for certain, tell if 逼 is a substitute for 屄 or not.

Baidu Baike gives no indication that it's a curse word.

But 懵X brings up almost four million results on Baidu &amp; 懵屄 has it's own fair-share of results.

Could it just be a happy coincidence?
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...)