I think the reason some people like to denigrate the quality of frum novels is to justify themselves for reading other material, which often contains inappropriate content (almost unheard of in the frum publications).
Even if we grant that there is a difference in the quality level, it doesn’t justify it, and I’ll tell you, there are plenty of intelligent frum people who very much enjoy frum literature. Ray’s Pizza might very well be better than Binyomin’s Kosher Pizza, but I still enjoy the kosher stuff.
Perhaps their minds weren’t spoiled by the spiritually harmful stuff which some of the complainers have indulged in.