Well you can't make an arguement about software sales strictly based on one game. In fact if you look at attach rates they are way higher on the Wii then the ps3, comparable as they came out around the same time. In fact if you look in japan the ps3's attach rate is quite troubling, selling far more consoles then games week after week, alot of people are probably buying it just for the blu-ray, whereas anyone who bought a Wii has no choice but to buy games, I don't imagine anyone is shelling out 250 bucks just to play one game, I expect even the most casual of users to at least purchase 4-5 games to justify buying the system, and the attach rate the Wii has had has pretty much backed that up.
As for Spiderman not selling well on the Wii it may just be competition, the ps3 has no other games in the top 20, meaning Spiderman 3 was all there is worth getting lately on the ps3, the 360 has 3 other games in there but also a much biger user base. The Wii has 5 other games on the top 20 which also shoots down your point about the Wii being proven a fad, and shows there was less room at the top for Spidey.
Of course the other titles are Nintendo made so you can argue the point of poor 3rd party game sales, but I still think it's not worth talking about until the high quality 3rd party offerings hit the system. After all as brought out Spidey was continually bashed as looking the worst on the Wii.
I also think another factor is the DS. Alot of kids who might of picked up the Wii spiderman probably own a DS and are consumed at the moment playing Pokemon. It probably is indirectly affecting sales.