It is perfectly acceptable to take the piss out of any religion/custom. It’s not acceptable to go up to someone in the street and do so but Islam is not off limits to humour, criticism or examination
I agree but I think there is an issue about how something is done and who it's directed at. He didn't 'take the piss' out of a religion, he took the piss out of vulnerable women. He didn't 'ridicule' a style of garment, he objectified and ridiculed the women wearing them, and it added fuel to the far right. As well as support for his stance politically, another response is that there are images of Muslim women being circulated all over Twitter with really nasty comments about them attached. And there are videos showing that the term 'pillar box' has been used by racists harassing Muslim women.
He is being investigated by his party in relation to the Conservative Code of Conduct for MPs I think these paragraphs:
'- lead by example to encourage and foster respect and tolerance'
' - not use their position to abuse, bully, victimise or harass or unlawfully discriminate against others'
Also if we go back to OP's original question, when talking about abusive behaviour there's often reference to a 'pattern of abuse' and I think that Boris Johnson has 'where racism is concerned. What is interesting is why when there were calls to discipline him in 2016, also for racist slurs, the Party refused to do so. I wonder if the only reason he's being investigated now is because of his position in the battle for what the Party will represent politically?