While the word "stereotype" can denote simple "cataloguing" or "categorization" of a concept, the most common connotation of the word implies an oversimplified and often biased mental image. In this sense, it generally means that the person is not actually thinking about the thing itself, but is rather using the conventional image that was received or formed at some previous time. Therefore I wouldn't say that it's any sort of prerequisite for mental functioning, and could even be seen as a hindrance to real "thought".
