So... because she never fought Wario without genies or robots in Wario Land, she can't be playable in Smash Bros.? Because it wouldn't be accurate to her canon, right? Is this the problem?
The fact that in the WL games, that she's basically the equivilent to Dr. Wiley doesn't discount her from being a playable character, though it is something that would be akward given her past history. In opposition, Mona's more apt to fight at close range, and the microgame idea can be implemented in such a way too. Its just that one character fits better as a playable character in this situation. (Though don't get me wrong Syrup could be playable, but it just wouldn't fit as well)
How is she not active? She's everywhere and she pretty much carries the (very basic) plot the entire way through. The only thing she didn't do was have a boss fight. And that's ignoring the other Wario Land games she's in (particularly #2).
She sold maps, and stole treasure without confrontation. She wasn't very active. Perhaps the problem here is that I'm using the word active in a unusual way though. What I mean by it is that she is not as inclined to action in Shake It, and rather just sits on the sidelines. In WL2, where she had her biggest role, she was more active, though she never got into any combat herself.
As for Wario fans LOVING the conflict between them, you really need proof? Look around the internet, preferably the same way you were supposed to look around for proof that people like Roy. Do you really expect people to not care about the only recurring villain in Wario Land?
She's the first recurring villain, but not the only one technically. (Poor Rudy. No one loves you. ;_; )
But anyways, I really don't know what you're refferencing. I've been checking around GameFaqs, and don't see anyone there LOVING the relation between Wario and Syrup. Is there any other type of forum out there that you're reffering to? Really now, the internet is a big place.
Considering ANY motion controls three years ago were revolutionary (hence the original name), I'm sure people would eat up what Shake It had, especially given WL's sales history.
I somewhat doubt that, given how mass appeal normally isn't for platformers any more, and how Smooth Moves was generally geared towards a wider audiance. If it had WL's sales history, then it would be doing better than it is right now though. None of the other WL games were launch titles after all.
I don't know how many times I have to say this.
Series sales =/= Individual character popularity
You can't accurately measure popularity of individual characters within a series by sales. At most, sales will tell you how popular the main character is. And even that's debatable, depending on the game and series. If you want to know who's important in an ensemble, you find the fans themselves. It's painfully obvious that you haven't done that.
Its true that sales don't say a individual character's popularity, but it does indicate the series popularity. In a case like this, it gives Mona a advantage over Syrup, though that's all it is, a advantage. Thus in something that's fairly even, I'd say the person with the advantage is more likely than the one who doesn't have it. And popularity through forums is quite a difficult thing to gauge, given the nature of most forums. At this point, there is no real "Who would you rather have as a Wario character in SSB4" threads going on around here, or any other place that I am aware of, so I really can't say the general fan's reaction, but I don't think you can either. As it stands, Syrup got kicked out of the WL games for a bit there, while Mona's been in nearly every WW game so far (The exception is the Wii version of WW DIY, but I think that's only because the characters used in there were all ones not used in the DS version) Now you're probably going to nitpick me judgeing characters popularity based on their attendance in their own series, but as far as I'm concerned its usually pretty faithful in series with reoccuring characters, since people want to see more of them. At the very least it seems a better idea than using the internet alone as a basis for popularity.