To a certain extent Brooks gets a pass on some of the extra hate that Lesnar got for being the first big name sports entertainment star to get into the UFC, plus all the favorable match ups (in terms of ranking and getting the title shot) in the opinions of the MMA fans. He brought a lot of pro-wrestling fans to the sport that were as initially frowned upon and sneered at as the TUF noobs of a couple years earlier.
Brooks doesnt get a lot of that hate because that barrier has already been broken. He does get extra hate, however, for being allowed in the UFC with the fighting background of any random mid-30s guy in most neighborhoods but having a famous name. Lesnar at least had a pedigree of wrestling champion that even the haters couldnt deny. When he said that he wanted to do it basically because it was on his bucket list, and is allowed to get away with it, it makes every other person who kind of had that on their bucket list and knows it will never happen kind of mad. Its supposed to be the UFC, the major leagues. Not Fight Club where you have a retired action star fighting a photographer for kicks.
He also gets extra hate from the fighters who have been training their whole life, giving up other opportunities to fight their way into the "Big Show" for whatever salary they can get just to watch someone get a bigger payday for being a celebrity.
So the situations are similar but a lot of the motivations are different, which might be why it seems bigger or more vocal.