There's no clear indication that it's popular. Just from the title alone, it's not a name that immediately rings a bell as a super - popular light novel. But it could be a hidden gem that only a small group of dedicated readers know about. Maybe it has a really unique story that just hasn't caught on with the mainstream yet.
Well, in light novels, they often have unique settings and characters. 'Kumo desu nani ga' could potentially be a story set in a fantasy world where 'kumo' plays a significant role. It might involve a journey of a kumo - like being, facing various challenges and making friends or enemies along the way. There could be elements of magic or special powers associated with the kumo concept.
Since I haven't read'murabito desu ga nani ka web novel', it's hard to give a detailed account. However, from the title, it could be that the story centers around a character who identifies as a'murabito' (a regular person). The 'nani ka' part implies that there is something going on with this person. It could be that they get involved in a strange or unexpected situation in the web - novel's world, like uncovering a mystery or facing a unique challenge.
A common theme is the clash of different worlds. Since it's a crossover, kazuma mikura often has to deal with the differences between his world and the world he crosses into. This can create a lot of interesting storylines.
I think it happens in 'Konosuba: God's Blessing on This Wonderful World!' But the details might vary depending on how you interpret their relationship development.