It is OK to criticize religious Jews for their failings. Hashem did it. Moshe Rabbeinu did. The Neviim did it. Chazal did it. Mussar is GREAT!!! החזק במוסר אל תרף נצרה כי היא חייך said the wisest of all men. Guard mussar b/c it is your LIFE!!!
We are not shy about admitting that even religious Jews have much to fix.
HOWEVER - a big however - this is provided that the goal is to give mussar to the people so that they change their ways. If the criticism is told to the Gentiles [or those Jews who live and think like Gentiles - like on secular websites] then it is an egregious sin. Sadly, one sees on social media that some religious Jews have a sport [and at times make a living] of telling the OUTSIDE WORLD how rotten we are. What benefit is there to this [besides getting a lot of attention etc.]? The people at whom the criticism is directed don't go to these sites and those who do read this generally don't appreciate how special Am Yisrael is so all they come away with is that we are the WORST of all people. This fans the flames of hatred.
We have little power to do anything about this. But we should know that there is a Judge who knows what is in people's hearts and what their motivations are. If their motivations are anything but pure then they are treading dangerous terrain.
ולמזהיר ולנזהר שלומים תן כימי נהר
