Thinking about it 3 different ways:
1) Raw software sales - Animal Crossing is going to be the best selling game on the Switch. 3D Mario All Stars is going to sell more than 10m and Age of Calamity might do it too. Of course, there are a lot more first party games that will sell in the 1-5m range. The only other year that was better for titles turning into raw software sales was 2017. In this category I give 2020 a B.
2) My personal enjoyment - I haven't played Age of Calamity yet, so this is somewhat of a guess. However my two favorite Zelda games of this century are BotW and Hyrule Warriors. I'm definitely buying Age of Calamity. I've been enjoying Super Mario 35 a lot. I am definitely going to buy the physical version of Cadence of Hyrule. There is also a decent chance I'll get Mario Kart Live and Clubhouse Games at some point. For personal enjoyment I'd have to say this year is tied with 2017. Nothing is as good as BotW from this year, but there are more first party games that I'm interested in. For this category A-.
3) Variety and originality - This year relied a lot on first party ports which is the worst thing I can say about it. (Although 2017 and 2018 also had a lot on first party ports.) However, I think Mario Kart Live has been the Switch's best original game so far. It easily beats LABO and 1-2 Switch. The only real contender for me is Mario+Rabbids, but I find the AR Mario Kart idea more interesting. Age of Calamity also gets some originality points from a storyline perspective. Nintendo is taking a cue from Marvel on this one. They are giving us an entire story trilogy on one system, and they can do it because they are partnering with another studio on Age of Calamity. So the Zelda trilogy is also a plus. However, the quanity of actually original titles this year brings this category down to a B.
Overall score: B+.