As a Wii U owner, no. It's not. And it's just a lazy way to make the year look better for the system than it is. I mean we got, what? Mario Kart, Pokken, Rayman Legends, Lego City Undercover, DQX (Japan), and I'm sure much much more.
Jesus, imagine if this had been a regular Nintendo console with BC. This year wouldn't be much better than previous years! With the only (major) exclusive being Mario. Which is cool, but currently it's "must buy" is a game originally intended as a Wii U exclusive. Which is, in my opinion, extremely pathetic.
It's a portable console, you can take all these games with you on the go... That wouldn't be possible on a backwards compatible console... Optical disk media and portable gaming are not a match made in heaven. Too easy to get damaged and the disk drive would take up too much space making the console too bulky to take on the go and leaves less room for components that make the Switch as powerful as it is (for a portable console).
And Mario Kart 8 Deluxe is a greatly enhanced port with 5 new battle modes and actual battle stages, all the DLC included for free, new characters and running at 1080p 60fps.
The Wii was also backwards compatible with the Gamecube, but it still got a launch port of Twilight Princess and it definitely helped sell a lot of consoles in the first few months. Breath of the Wild on the Switch got released on the same time as the Wii U version anyway, so it's a brand new game and not "just a port" as you make it out to be. It's also superior to the Wii U game and it looks A LOT better in handheld mode than on a big screen TV. Runs smoother as well.
So no, the only major exclusive is not Mario...
It's The Legend of Zelda: Breath of the Wild, Mario Kart 8 Deluxe, Arms, Splatoon 2, Mario + Rabbids: Kingdom Battle, Pokkén Tournament DX (adds five new characters), Super Mario Odyssey and Xenoblade Chronicles 2.
Also, the Switch only sold 13 million consoles during its entire lifetime, which means a lot of people didn't get to enjoy these great Wii U games. It would be stupid of Nintendo to not make ports of some of these games.
It's not like these remasters take away resources that would've gone to brand new games instead...
Pokkén Tournament DX is developed by Bandai Namco and the extra content comes from the Arcade version anyway...
Mario Kart 8 Deluxe was developed by the Mario Kart team and they recently released a brand new IP as well... But for some reason you seem to ignore Arms.
The Legend of Zelda: Breath of the Wild port to the Switch probably delayed the game, but it was well worth it. Hard to argue against sales numbers and critical reception. They're also releasing a story expansion this holiday season...
Lego City Undercover was developed by a third party studio and it got ports for all current-gen consoles, so it would make no sense to leave the Switch out. It probably sold better on the Switch than the PS4 anyway. And Switch will get all the brand new Lego games anyway, like Lego Worlds...
Rayman Definitive Version is being developed by Ubisoft and it's not like Ubisoft is not supporting the Switch with brand new games... So far they've announced Starlink: Battle for Atlus, Steep, Just Dance 2017 & 2018 and Mario + Luigi: Battle Kingdom.
Dragon Quest X is developed by Square and so far Square has been one of the best supporters of the Nintendo Switch with games like Dragon Quest Heroes 1 & II, Dragon Quest XI, Project Octopath Traveler, I Am Setsuna, Lost Sphear, etc.