I don't know how to feel about this. On one hand, you are right about Splatoon not being overrated, on the other hand, you are wrong about it "sell like trash."
Honestly though, it is going to sell well compared to the likes of Codename Steam and W101 for two reasons.
1. People are actually talking about it. Just, look up all the youtubers who decided to join in the testfire, and expressing their thoughts on the direct, etc. They actually cared enough about it unlike Codename Steam which barely anyone batted an eye.
2. People are enjoying it after playing it. Like people are really happy. Some people who don't enjoy shooters at all say they enjoy it. I don't hear many negative things from reviews. The only thing I did hear was from IGN about the lack of voice chat,but even then they said they loved it. This unlike Codename steam where people didn't like the artstyle or how long it takes for the enemies to take their turn, or w101 apparently being complicated.
Honestly, Nintendo actually did well advertising it for a change. With the tumblr, an actually good direct, and the all the events, people have found ways to get excited for this game. Even with all the hiccups along the way. It may not do amazing, but I can see it doing well enough for something like a sequel.
Lastly, overrated overall is a silly word. It isn't actually criticism. All it is is "I don't care for this game, a lot of people do, so therefore it sucks." I mean, really the word overrated exists just so people can have fights. This isn't really directed at you though.