I don't agree - I use dumb very occasionally. It isn't meant as an insult.
You probably use it on posts that a neutral observer could agree are dumb. The 'dumb' trio leaves it on well argued and thoughtful posts that certainly don't deserve that label. They use it to insult and dismiss. It is the equivalent to f*** you.
BTW: we live in a world where intentions don't matter. perceptions do. so if people perceive something to be a deliberate insult then it is a deliberate insult. Now if you want to revisit the standards brought to us by progressives then we can but that is a separate discussion.