I think there's never been big Direct between March/April and E3.
Well, they actually did several times in the past, but it has gotten rare recently. But I think that's also due to making less Directs in general now. For instance, there were 30 Directs in 2013 between the different regions, but only 17 in 2018 - and that's bloated through 5 Fire Emblem Heroes Directs.
However, in April/May, Nintendo generally showcases 1-2 games which are normally to be released in June or July, so it's normal that SMM2 gets that treatment, too. Just look at those directs in the last years:
2018: None, MTA direct was in early March already
2017: 2 Directs for ARMS and Splatoon 2
2016: Culdcept Revolt (Japan only)
2015: 2 Directs for Xenoblade Chronicles X and Splatoon. Also, a general Direct on April 1st.
2014: 3 in April, with SSB, Tomodachi Life and Mario Kart 8 in quick succession
2013: Monster Hunter 4 and Phoenix Wright (Japan only), also a general Direct on May 17, 3 weeks before E3
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454