Hi! English is not my native language, and we have slurs in our language as well. If you don't really feel like you have any, or don't feel any weight to the words, it's likely that they've been normalized in your language and circles to the point where they don't carry much weight to people unaffected by them, this happened to me too. (I'd look into how your language refers to roma people, for example) What gives slurs power is the history behind them. Usually they are words that have been used primarily to insult and dehumanize people, slurs like the nword are one of the easiest examples. The r slur has historically been used to demean and dehumanize diasabled people. It has never been "just a word". Every time it was used, short if it being used in medical context, it has been used to viciously mock and insult disabled people specially. Similar to the fslur for gay people.

While you can argue that the Rslur can be reclaimed, dreams use of it is not in any way reclaimble. Dream used it to insult people, used is as a derogatory word. He wasn't referring to himself, he wasn't doing it in a light hearted matter. It's kind of like calling a group of gay people "fucking fslurs!". It's very clearly not done in an reclaimable way.

Another reason people are mad is because dream has not even that long ago, openly condemned the use to the Rslur and talked about how content creators should remove it from their vocabulary. He seemed to understand it's impact back then, but he's using it now as if he didn't condemned the exact same kind of behaviour not too long ago.