The connection might be that pulp fiction often has characters that can be seen as'square'. So it's like saying don't be like those uninteresting characters in pulp fiction.
'Don't be a square' is like an admonition against being too conventional or rigid. 'Pulp fiction' has that connotation of being a bit trashy but also really engaging. Maybe it's saying don't be the stodgy person in the world of this edgy, somewhat tawdry pulp fiction. It's about not being the uninteresting one when there's all this wild, fast - paced pulp - like action or story - telling going on.
Well, 'square' can mean someone who is old - fashioned or not cool. And 'pulp fiction' is a type of cheap, mass - produced fiction often with exciting or lurid stories. So it might be saying don't be such an uncool person in the context of the world of pulp fiction.
The phrase 'don't be such a square' is a bit of a put - down, suggesting not being overly conventional. 'Pulp fiction' has its own set of tropes and characters. Maybe it's a way of telling someone not to be as one - dimensional as the common characters in pulp fiction, to be more open - minded and exciting.
I think it might mean don't be too conventional or stuck in a certain mindset when it comes to pulp fiction. Maybe it's urging you to think outside the box.
It's kind of a slangy way to tell someone not to be so old-fashioned or uncool, especially in a certain context related to something like a story or situation.
Basically, 'don't be such a square pulp fiction' is a way of saying don't be so stuck in the past or conventional. Maybe you're not getting with the exciting or unconventional aspects of a fictional narrative or real-life scenario.