I will say it was not a fad but had fad like properties.
Not a fad because some people bought it because it had games that they liked and/or were Nintendo fans. These people bought the Wii to play games SSBB, the Metroids, Zelda SS, maybe even the Condiut (lol), etc. These people bought the Wii because they liked the game selection, etc. much like people have been buying consoles for decades. These people gave the Wii a "typical" sales curve.
The fad-like part comes at the increased sales from people who wouldn't have bought a Wii ever, if not for Wii Sports. They saw Wii Sports or here that the Wii was new and cool and not the typical gaming device and were excited about it. This type of Wii purchaser ran out during 2006/2007 and picked it up in droves. Unfortunately, the Wii didn't offer them too much especially after a few years. Some of them bought WSR, Wii Fit, Mario Kart, or SMBW. But, once the Wii's initial "wow" wore off on them and the Wii was no longer something no one had ever seen before, it turns out they weren't all that interested in the games. They were simply interested in the Wii because it was new and cool and offered some entertainment for a while.
So, for these Wii purchasers, I'd say the Wii was a fad. They bought it mainly for Wii Sports and probably only own the bundled games that came with the system, but once the novelty of the Wii wore off, they didn't find it useful any more and the hype was gone. No one was telling their friends about the Wii anymore because there was nothing to tell or everyone had already heard it before.
So, because it had fad-like properties for some of the purchasers IMO is why it has a weird sales curve. The people who bought the Wii because it was a Wii gave the sales curve the usual 2-3 year gains followed by slow and steady drop off. The "I bought a Wii because it was cool" people caused the Wii to have a HUGE initial spike in sales followed by a HUGE drop off when no one bought it anymore because it was no longer a novelty for them.
Just my opinion :)