It's cause a large part of Sherbros had, and still have a (semi) boner for Costa, and hate Izzy. Part of them are in the closet, they're in awe of big muscly manly men. They drool when they see Costa's physique (which is impressive I have to admit) and their brain cannot compute with non-traditional manly men actually being straight up killers that beat their favourite fighters.
Of course Izzy has given them plenty of ammunition with the dog thing. He's making it very hard to like him still. All that pent up rage and frustration from when he was bullied is spilling over far into his now thirties, and he's basically just a petty nerd that just so happens to be a world class fighter.