Mike is not good at English（改成同义句）

Mike is bad at English.

Mike doesn't do well in English.

Mike do badly in English.

以上三种提法都可以。

be good at反义词是 be bad at
同义词是 do well in

do well in 的反义词又是 do badly in
所以一楼的回答很正确。

Mike is poor at English