Splatoon 2 is better than one in almost every way. Also shame on those saying it is a "lazy port" as if they feature all of the same maps and single player. By that logic, Uncharted 3 was just a lazy port of Uncharted 2. Same gameplay and visuals, different story and setting. I mean come on guys, is that the best you can do?
Now OP, what I think may be happening is that since Splatoon 1 was a fresh new IP at the time, you were probably blown away by how different the game was. Splatoon 2 however probably just felt like much of the same because you were looking for that fresh experience that Splatoon 1 gave you. The problem with that logic is that had they changed it much more than they did, it would no longer be a Splatoon game. People would say they changed too much and criticize it for that instead.
Never go into a sequel of a game expecting drastic changes, it most likely won't have them. What they will have is maybe a few new modes, new multiplayer maps, and SP story mode. This is nothing new.