Well dippy means dumb yes? Dippy to me also means full of themselves and slightly naive. I think the slashers you're looking at have this idea of teaching morals. The people who survive horror movies are the self-aware ones who don't waste their life away drinking and having sex. Things that distract you in a time when you need to know everything you can about your surroundings.
Also I know I love to see the ditzy blonde go first. Chances are, so do the majority of viewers. Ratings and all that jazz.
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