I think mainstream 3rd party games like Minecraft, Rocket League, Fortnite and Overwatch certainly helped to attract a lot of players. These are the games that the people talk about a lot so the Switch became a lot more attractive because not only can you play awesome 1st party titles but also popular games that all your friends play. I think these games were quite important in the Switchs' history because they contributed to a diverse and interesting library. The upcoming Apex Legends might be another game in that same regard.
90% of 3rd party games are shovelware on the eshop, though, and many pump out their stuff just because they know that the Switch is hot right now, and at the end of the day it's still Nintendo who does the heavy lifting with their own games, so I'd say 3rd parties are certainly helping, but not a major reason for the Switchs' success.
