I'm not surprised it's bombing. I'm an old school '80s Joe fan and this is pretty much the first SSC Joe figure that seemed extraneous and unnecessary to the line (the Crimson Commander coming closest previously, but at least it was still a nice combination of two iconic Joe elements). This was the first one I considered not getting.
First of all, the Ninja Viper was a very late addition to the figure line after no one but the hardcore collectors or kids were still into it. It was a 1992 mail-in, a couple years before the line died. I think it got notoriety among collectors because it ended up pretty rare, since there were few people around to mail away for it at that time. But it doesn't have much interest as a concept or a character, just as a figure that ended up rare.
At least it was a repaint of the very iconic 1984 Storm Shadow mold, which is why Hasbro's recent 25th anniversary remake of it was disappointing, having used the mold inspired by the 1988 Storm Shadow instead. This Sideshow version likewise is an almost unrecognizable version of the character, especially since the color has been changed. This is the closest they could have gotten to just making up their own G.I. Joe figure and still be able to claim it was based on an old character.
But, really, there's no nostalgia factor for this character. It comes from the flourescent era of the Joe line that no one much liked and just by accident became a rare and expensive figure. It's still a figure only completists care about. A more accurate version from Sideshow would have looked goofy of course. There really was no right way for Sideshow to do it so they shouldn't have done it at all. It would be kind of like them doing a rare Ewoks or Droids cartoon character in the Star Wars line. It might be a rare, expensive figure now but only because the supply is so low, not because there are a lot of people demanding it.
Now a black ninja would have sold like gangbusters. Granted it has nothing to do with the Joe mythos outside of being a rare toy, but at least it was one from the '80s during the heyday of the line that just happened to be exclusive to a foreign country. And black ninjas are just cool.