I mean, it's Screen Rant. Minimal effort clickbait is their entire schtick. How else could a Mario Kart, Smash and a mainline Mario widely considered one of the best games on the system be considered underrated.
Also, if we're talking underrated Wii games, along with those mentioned already, of the top of my head, I'd like to present for your consideration:
- A Boy And His Blob
- Zack & Wiki: Quest for Barbaros' Treasure
- Muramasa The Demon Blade
- Trauma Team
- Shiren the Wanderer
- Deadly Creatures
- Disaster: Day of Crisis
- Little King's Story
- Nights: Journey of Dreams
- Rhythm Heaven Fever
- Wario Land: Shake It
- Rodea The Sky Soldier (but only the Wii version)
- Fragile Dreams
- Batman: The Brave and the Bold
- Geometry Wars: Galaxies
- Excitebots: Trick Racing
- Klonoa: Door to Phantomile remake
- Kororinpa
- Dawn of Discovery
- Dead Space: Extraction
- Battalion Wars 2
- Cursed Mountain
- Dokapon Kingdom
- Pandora's Tower
- A Shadow's Tale
- Tatsunoko vs Capcom
- Sky Crawlers: Innocent Aces
- Rune Factory Frontier
- LA Machine Guns
- Another Code R (currently EU/JP only)
- Dragon Quest X: Awakening of the Five Tribes (currently JP only)
- Captain Rainbow (currently JP only)
And probably some I'm forgetting. Maybe not all the most mindblowing games of all time, but imo definitely underrated, and worth a fiver or tenner given the opportunity. So just pick 10. Any 10. Will make for a better list.