Pulp fiction just means works published in pulp magazines or similarly disreputable publications. Some was bad, some was great. Many acclaimed authors wrote for the pulps:
https://en.wikipedia.org/wiki/Pulp_magazine#Authors_featured
https://en.wikipedia.org/wiki/Pulp_magazine#Authors_featured
Pulp is, by association, low-quality writing.
Pulp is also, by allusion, a particular genre/structure mirroring the dominant topics and styles of the original magazines - regardless of its quality.