No. Splatoon can sell 5-6 million WW lifetime. 1 Million in japan and heck maybe even 1.5 mill in Japan. It's not going to outsell or come close to Dragon Quest or Monster Hunter numbers tho.
Did you know Splatoon is the best selling home console game between ps4/xbone and wiiu in Japan? Did you know Splatoon sold 1.5 million units on a 3.3 million userbase (Japan)? Do you believe splatoon 2 won't sell more on switch than it did on wiiu?
That being said, dragon Quest 7 warriors is the most sold dragon Quest game on 3ds with 1.3 million units...the first splatoon already beat it.
Monster Hunter 4 sold 3.4 million units. I don't think splatoon 2 will surpass it but I think it can definitely get close. I think you underestimate how big splatoon is over there.