I think that she was included primarily because Sakurai had an idea on how to use her. HOWEVER, if it were not for the sales of Wii Fit, can we say that she would've occurred to Sakurai at all?
People here are hardcore gamers, and not interested in games like that, so most of us would never have thought about her.
I don't think Sakurai would've included her because of Wii Fit's sales if he didn't have any good idea about how to use her and didn't think she would make an interesting/fun/out there addition to the game. But I think without her sales, she would've been much less likely to have been considered and decided upon.
Also, when you also think about the notion that the game is for Nintendo's all-stars, well, it's her sales that make her fit in in that sense. I don't think they're irrelevant.
ETA: I don't think, however, that she was added to promote Wii Fit or because they think she'll bring in tons of sales because of how popular Wii Fit is. The audiences for Smash Bros and Wii Fit are VERY different, and I highly doubt anyone will buy Smash Bros because their favorite trainer is in it. I also doubt appearing in Smash Bros will make much of a difference to Wii Fit's sales. I don't think she was added as some kind of money grab, at all.
I think it's relevant in the sense that she's not unheard of, she represents something significant about Nintendo and her sales make her plausible to refer to as an "all-star." If Wii Fit had been a flop, I see basically no way she would've been included, since she would have represented a failure of Nintendo's, been unknown, and most certainly not an all-star. There's a reason you don't hear much about Virtual Boy and it's not because Nintendo doesn't remember it existed.