I hate people who keep their mouth open while they chomp on their food.
牛や馬は口を開けてむしゃむしゃ食べることから、次のように言えます。
ーI hate people who eat like a cow/horse.
「牛/馬のように食べる人が嫌いだ」=「口を開けて食べる人は嫌いだ」
ーI hate people who keep their mouth open while they chomp on their food.
「口を開けて料理を噛む人は嫌だ」
to chomp on で「むしゃむしゃ食べる」
ご参考まで!