Famitsu published the top 500 of 2013 game sales (in Japan ofc) a while back and of of course Game & Wario included.
For funsies, I included all other Wii U game listed in there to see how G&W compares. It's very unlikely the game has sold much more since then (Note that Famitsu did not start tracking digital number until sometimes in 2014, but those are around 10% for most games).
09. [WIU] New Super Mario Bros. U # <ACT> (Nintendo) {2012.12.08} (¥5.985) - 584.479 / 965.794
10. [WIU] Wii Party U # <ETC> (Nintendo) {2013.10.31} (¥4.935) - 518.766 <---- game was bundled with console.
15. [WIU] Super Mario 3D World <ACT> (Nintendo) {2013.11.21} (¥5.985) - 410.041
39. [WIU] Pikmin 3 <ACT> (Nintendo) {2013.07.13} (¥5.985) - 222.909
61. [WIU] Nintendo Land # <ETC> (Nintendo) {2012.12.08} (¥4.935) - 168.580 / 382.954
92. [WIU] New Super Luigi U <ACT> (Nintendo) {2013.07.13} (¥3.885) - 100.231
100. [WIU] Taiko no Tatsujin: Wii U Version! # <ACT> (Bandai Namco Games) {2013.11.21} (¥5.480) - 96.076
114 - DQX v1 - 81,926 / 81,926
123 - DQX v2 - 77,672 / 77,672
124 - Game & Wario - 77,323 / 77,323
149 - Lego City Undercover - 60,618 / 60,618
156 - Wii Fit U (w/ Wii U bundle?) - 58,769 / 58,769
159 - Monster Hunter 3 Ultimate - 58,078 / 224,986
162 - Legend of Zelda: Wind Waker HD - 56,059 / 56,059
215 - Tank! Tank! Tank! - 36,189 / 36,189
261 - Mario & Sonic at the Winter Olympic Games - 27,279 / 27,279
281 - Warriors Orochi 3 Hyper - 24,841 / 24,841
295 - Resident Evil Revelations - 22,793 / 22,793
298 - Simple Series Wii U Vol 1 - 22,414 / 33,818
306 - Zombi U - 21,748 / 43,290
307 - Epic Mickey: Power of 2 - 21,690 / 21,690
309 - Tekken Tag Tournament 2 Wii U edidtion - 21,496 / 35,995
312 - Fist of the North Star: Ken's Rage 2 - 21,255 / 21,255
332 - The Wonderful 101 - 19,549 / 19,549
338 - ESPN Sports Connection - 19,396 / 30,431
433 - Call of Duty: Black Ops 2 - 12,850 / 21,471
465 - Pokemon Rumble U (DL card) - 10,955 / 10,955
483 - Assassin's Creed III - 10,140 / 17,451
504. [WiU] Disney Infinity (Bandai Namco Games) (2013/11/28) - 9,458
509. [WiU] Ninja Gaiden 3: Razor's Edge (Koei Tecmo) (2012/12/08) - 9,370 / 15,486
514. [WiU] Batman Arkham City: Armored Edition (Warner Entertainment Japan) (2012/12/08) - 9,233 / 13,837
570. [WiU] FIFA 13 World Class Soccer (EA) (2012/12/08) - 7,809 / 12,410
574. [WiU] Yakuza 1 & 2 HD for Wii U (Sega) (2013/08/08) - 7,753
596. [WiU] Romance of the Three Kingdoms 12 (Koei Tecmo) (2012/12/13) - 7,270 / 11,575
619. [WiU] Sonic Lost World (Sega) (2013/10/24) - 6,502
624. [WiU] Disney Epic Mickey 2: The Power of Two (Spike Chunsoft) (2013/09/26) - 6,420
691. [WiU] Call of Duty: Ghosts (Subtitled) (Square-Enix) (2013/11/14) - 5,172
699. [WiU] Need for Speed Most Wanted U (EA) (2013/03/20) - 4,963
703. [WiU] Monster Hunter Frontier G Beginner's Package (Capcom) (2013/12/11) - 4,911
707. [WiU] Mass Effect 3: Special Edition (EA) (2012/12/08) - 4,777 / 6,526
803. [WiU] Sniper Elite V2 (Ubisoft) (2013/07/18) - 3,454
804. [WiU] Splinter Cell Blacklist (Ubisoft) (2013/09/05) - 3,452
810. [WiU] Rayman Legends (Nintendo) (2013/10/17) - 3,415
836. [WiU] Injustice: Gods Among Us (Warner Entertainment Japan) (2013/06/27) - 3,029
906. [WiU] Assassin's Creed 4: Black Flag (Ubisoft) (2013/11/28) - 2,390
914. [WiU] Rabbids Land (Ubisoft) (2013/06/06) - 2,335
989. [WiU] F1 Race Stars: Powered Up Edition (Codemasters) (2013/06/27) - 1,744
Some games released after 2013:
Mario Kart 8 - Around 890k
Smash Bros Wii U: around 500k
Captain Toad: Near 115k
Mario Party 10: Near 120k
Hyrule Warriors: 71,550 in its opening week, can't be arsed to find anything after that.
So yeah, Game & Wario's sales aren't good at all but they *aren't* as horrible as I remembered them being. I was surprised to find it outsold Wind Waker HD and Sonic Lost World, even if both franchises have been historically less popular there.