be good at的反义词可以是：be bad/poor at..； be weak in...；表示不擅长或在某些方面做的不太好。

be bad/poor at sth. 不擅长某事
be bad/poor at doing sth.不擅长做某事
be weak in sth. 不擅长某事
be weak in doing sth. 不擅长做某事

【例句】
Mr Green is bad/poor at sports.
=Mr Green is weak in sports.
格林先生不擅长体育。
Tom was bad/poor at Chinese.
= Tom was weak in Chinese.
汤姆不擅长中文。 
Mathew is weak in public speaking.
=Mathew is bad/ poor at public speaking.
马修不擅长公众演讲。
Andrew is bad/poor at swimming.
=Andrew is weak in swimming.
安德鲁不擅长游泳。