It's going to be down Year-on-Year on a few weeks or a couple of months, isn't, though? As far as I remember, that is the case at least for Japan.
You may argue that are games to launch and the holiday season could be bigger for the Switch, but that still remains a more distant prospect, yet to be proven, and being down YoY is closer to reality right now.
May and June were the worst months for Switch last year by a substantial margin. Switch is currently up year-on-year in Japan and has sold 37k per week during April 2018, while it sold an average of 32k in May 2017 and 28k in June 2017. Additionally, the early part of May 2017 was boosted by the increase in supply thanks to Mario Kart 8 Deluxe, given that Switch sold 48k in the first week of May and 27k in the final week. Switch currently has a lead of 56k. In order for Switch to be down year-on-year at the end of June, its weekly sales must fall below a 24k average for the period, which is incredibly unlikely given historical trends.
Looking outside Japan, Nintendo's own shipment data shows that more Switch units were shipped in Q1 2018 than in Q1 2017.
And according to VGChartz, Switch's sell-through was up year-on-year more than 700k as of the middle of April.
The only region in which your argument could be made is the United States, because Switch was down between 50k and 100k for January-March, but even this would be shortsighted, given that Switch had the second best launch month of any video game hardware ever in the US. Just like in Japan, May and June 2017 were Switch's worst months in that year, and it will easily be up YoY by the end of Q2 2018.