NH's success wouldn't really be relevant, but Tom Nook himself is a popular hit character and the previous mascot(and far more important to NH than Isabelle anyway). They could choose him not to really promote NH, but because he's a notable character with his own merits. How Sakurai implements him is where the question would be. Nintendo isn't choosing the movesets, after all. So it's a matter of how he thinks he could be unique, and the timing would easily allow Sakurai to look at new stuff from NH to work in.
It's possible. The Spirit Event is directly meant to promote NH. Tom Nook wouldn't be really for that, though he could easily represent the game if Sakurai sees good materials to spice him up from Villager and Isabelle.
Not every character is chosen for the sake of promotion, really.