How to say "trash talking"?
In this case, what the American said was more a 風涼話 (the game was already over)

風涼話
sarcasm; cynical remarks (made after the fact)

As for trash talk:

trash talk
disparaging, taunting, or boastful comments especially between opponents trying to intimidate each other

It is similar to 吹牛皮 (boasting), 耍貧嘴 (babble with cynical words)  or 耍毒舌 (taunting with mean and provocative words)
牙斬斬 ; 狗噏 in Cantonese:

More importantly, the purpose of 'to trash talk' is 'to trash your adversary'
The most accurate translation of 'trash talking' should be: &quot;自我吹擂，貶損對方 &quot; (boast oneself and disparage the other)

A O wrote:

what would be an equivalent translation of &quot;not even close&quot;, in a joking, trash-talking context?

It would be '差遠了'