It doesn't mean "really good" just "good".
While you can use this construction of "good at -ing" I think it sounds smoother to use the noun form of the action verb.
You are a good dancer.
You are a good writer.
You are a bad singer.
Etc.
2015年1月28日
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Yes, you can say both of those - 'good at writing' or 'bad at writing'.
You can also say good/bad at for other things that people do, for example, 'good at dancing' or 'bad at singing'.