“Fugly” is a vulgar slang adjective as far as I know, and I wonder how common it is, and how do people react when they hear that word. Native speakers are appreciated if they share their opinions.
“Fugly” is indeed both vulgar and slang. When I was growing up it in Chicago, it was understood as a combination word built of f***-ugly, where the first word is one of the proverbial words you can’t say on the radio.
As a combination that was often used on the playground, it has less impact than the core four letter word. But Horatio’s description of it sounds about right – a direct insult.