Given that there are actual levels like in SPM, it's probably more linear than the first two PM games.
Lol, the first two Paper Marios are a textbook example of linear RPGs. Chapters have a clear cut progression with ton of mandatory event flags and no alternate paths (and hell, three of TTYD's chapters lock you in until you complete them), there are no choices with any actual consequence, no storyline variations, etc. For all it's fault, all SPM did with the level system was reducing (though not eliminating) backtracking- and lessen the opportunity for more varied NPCs dialogue, though if that means not suffering through another Twilight Town, I can live with that.
If anything, Sticker Star has the
potential to be slightly less linear than its predecessors, as the map system could allow you to choose one level to do over another, do a "world" in the order you like, etc-not that the promo material released so far shows any of this.