'Pulp' refers to 'wood pulp', as in cheap and without much substance. Though you're right, judging something as objectively good is a generally hard thing to do, especially when what you are really trying to do is guess what people in 100 years will think of as good.

Pulp is a genre. One of the characteristics of the genre is that it is written to sell (rather than eg. to achieve litterary recognition or to explore the limits of language or whatever). But just because something is popular does not make it pulp.