For obvious starters, Jamze himself is an extremely bland and one-dimensional bully. He is nothing more than a basic cool dude, which is generic and lazy in a show that has interesting characters with unique designs and personalities, not even his voice sounds different to a clichéd "teenage slacker from the 90s".
Even his name is trying way too hard to be rad and is very obvious, as he's called "Jamze Withazee".
Jamze is also an extremely unlikable character, he gets Mac to stay with him at a rock, which nearly causes Bloo to get adopted and consider him not cool.
Perhaps the biggest and most infamous reason as to why Jamze is so hated is what he does near the end of the episode after Mac stands up to the kids when they pick on Bloo, instead of apologizing to Mac when he stands up to the kids as they laughed at his best friend, Jamze basically betrays Mac by calling him a nerd and laughing like a maniac, causing all of the kids to laugh at both at Bloo and Mac, this is one of the cruelest moments that a character/villain has ever done.
As mentioned, his voice acting is clearly trying to make him hip, which will most likely only aggravate the viewer.
His behavior makes him much like Mac's brother, Terrence, who is also similar like him while lacking the wild behavior.
Jamze never gets punished for his horrible actions, nor does he even feel remorse at all or apologize to Mac, making him a karma houdini.
Despite his voice acting not even also sounding like different to a clichéd "teenage slacker from the 90s" and being trying to hip and will also most likely only aggravate the viewer as mentioned above in WH'sAUJ#1 and WH'sAUJ#4, Phill LaMarr did do an awesome job voicing Jamzee.
Like all of the characters in Foster's Home for Imaginary Friends, his design is also pretty creative to look at.
His appearance, "cool" persona and name is likely an homage to the late James Dean.
Because of his poor reputation by fans, due to his personality and his infamous moment near the end of the episode, he has been written as a non-canon character in the Foster's Home for Imaginary Friends series, as he only appears in this episode and is never seen or mentioned again, including the episode itself.
He is the second character to be written as a non-canon character in the Foster's Home for Imaginary Friends series, the first of which, of course, being Bendy.