They were really overvalued in my opinion. Even if a lot of fans are still happy with them, their presentation for E3 wasn't strong. They didn't bring hardly any games, they lack big new AAA exclusives, they lack big third part AAA support (and will clearly not get it headed into the future at this point) and Retro's absence, Yoshi's delay, and Fire Emblem's delay smell fishy.
People still seem sensitive about Smash, and while it's clearly labeled as a new game with tweaked mechanics and updated character models etc., it looks strikingly similar to that game that came out a few years ago. As their big game along with Pokémon for the whole year of 2018, that's pretty lackluster when you look at how high they were valued so it only makes sense it would drop.
Personally as someone who played mostly on Switch last year, I have hardly touched it this year aside from when playing games that came out last year, so as a fan I'm pretty dissatisfied as well.