Concept for 'too much exposure to a bad idea can make it seem normal'?
Are you thinking of desensitization?

From wikipedia: In psychology, desensitization is defined as the diminished emotional responsiveness to a negative or aversive stimulus after repeated exposure to it. (https://en.wikipedia.org/wiki/Desensitization_(psychology))

Desensitization seems to match the question as posed in the title, but maybe not the question as posed in the text.  Desensitization occurs based on mere repetition.  In the body of the question, it seems like you're referring to repetition that takes place in the context of otherwise normal (non-negative or aversive) context, so that the once-negative subject acquires some of the normalcy of the context.