Famitsu Sales: Week 51, 2020 (Dec 14 - Dec 20)

[Farsala]

[NSW] Mario Kart 8 Deluxe (Nintendo, 04/28/17) – 44,450 (3,388,984)
[NSW] Super Smash Bros. Ultimate (Nintendo, 12/07/18) – 33,475 (3,959,987)
[NSW] Hyrule Warriors: Age of Calamity (Koei Tecmo, 11/20/20) – 22,648 (278,612)
[NSW] Pokemon Sword / Shield (The Pokemon Company, 11/15/19) – 21,081 (3,852,055)
[NSW] Splatoon 2 (Bundle Version Included) (Nintendo, 07/21/17) – 19,871 (3,656,361)

MK8 Deluxe still outselling the 3 above it, but the progress is slow. Hyrule Warriors, a brand new game only selling half of Mario Kart 8 Deluxe.

Last Year for comparison.

01./01. [NSW] Pokemon Sword / Shield # (Pokemon Co.) {2019.11.15} (¥5.980) - 288.199 / 2.744.658 (+24%)
07./07. [NSW] Mario Kart 8 Deluxe (Nintendo) {2017.04.28} (¥5.980) - 37.835 / 2.618.126 (+69%)
09./08. [NSW] Super Smash Bros. Ultimate # (Nintendo) {2018.12.07} (¥7.200) - 34.085 / 3.417.667 (+61%)
12./12. [NSW] Splatoon 2 # (Nintendo) {2017.07.21} (¥5.980) - 19.662 / 3.231.666 (+76%)

MK8 vs Pokemon- 126,532 to 463,071, Pokemon just released so it gained YoY.
MK8 vs Smash- 799,541 to 571,003, Only ~220k gained for MK8
Mk8 vs Splatoon 2- 613,540 to 267,377, 350k gained for MK8, realistically the only game MK8 can pass next year,

Mk8 will always have 2022 to outsell Pokemon and Smash though.