这个句子不对。

[emoji]这 and 这个 always mean the same. You can also say "这不好。" My native language is Chinese but I'm not a linguist. In Chinese, costumary usage is more important than grammar.And this sentence is very colloquial, so don't be too entangled into the details. By the way, 很高兴你喜欢中文！

This sentence is not wrong literally. In this sentence, 个 is no longer a measure word, 这 and 个 combined together to be the subject.

“这个不好”means " This one is not good." “个” is a pronoun in this sentence. “这个不好”是简略的讲法，甚至还可以更简略讲，”这不好” 完整的句子是像这样，”这个学生的成绩不好”

Both of them.