בער does not mean stupid. It comes from the verb לבער to destroy. It means someone who is stupid and malicious. It is what is referred to nowadays as criminally insane. That means people that hate Musar (and they are many) are criminally insane.
The path of Musar is almost unknown nowadays. So at first I would like to say what is.
The common idea is that Musar is good for orientation like on your first day of university. People assume it is good to help you find yourself around in the value system of the Torah. That might in fact be one good use of it. But it was not the idea of Reb Israel Salanter.
The idea of Musar --learning medieval books of Ethics-is in the view of Reb Israel Salanter a way of coming to human perfection. He advocated learning it many hours every day for that purpose. Before I got married I took that idea very seriously and after I was married also. The responsibility of having and raising children weighed heavily on me to the degree that I felt I needed some kind of help from Heaven that came with making progress in Fear of God and good character.
Eventually this became connected with Litvak yeshivas. But the original idea of Reb Chaim of Voloshin the institution of a independent yeshiva as an institution in itself had only begun 60 years before the Musar Movement got started.The second such institution to come into existence was the Mir. And there was resistance to accepting Musar as part of the curriculum.
Eventually Musar became accepted in almost all Litvak yeshivas but not to the degree that Reb Israel Salanter was advocating.
My own experience with Musar was powerful. But I also can see the dangers of getting off track that happens to many. But at least for me Musar seemed to be a catalyst for the Divine Light אור אין סוף. So in spite of my misgivings about the whole thing, I think I would have to go along with Reb Israel Salanter on this issue. The benefits seem to outweigh the potential dangers.
I also heard from Eli Spoduk that he was unsure of what Torah was all about until he got to the Musar books of Rav Moshe Haim Luttzato. That is just one example of how important Musar is. Besides that it seems clear that people that get involve with Torah without Musar inevitably get very anti Torah world views.
What seems clear is that a good dose of Musar every day is indispensable and its value is independent of yeshiva. Authentic Litvak yeshivas are in any case rare. Most yeshivas are country clubs for chatting and blowing the breeze.