Yes-ish, in the same way you could describe Sushi to someone as "mainly rice"
At it's core it is indeed a platformer with puzzles, stickers, skateboards, you run around, jump, grab onto things, swing around, pull things, push things. But I think what really makes it stand out is all the tools that are used in a level are available right there in the game to you, you see a massive skateboard jump a ravine and think it's cool, you can recreate it and improve it if you feel the urge.
So consider it more perhaps a level editor, creature editor, item designer, platform game. People for example have already re-created side scrolling shooters in it, a physical calculator (has to be seen to be believed, that person had too much time on there hands), tetris, and more
So yes, it's a platform game, but to just consider it that is doing it a dis-service.