Most people against fortnite being on switch are just the typical "BUT MUH OPINION MATTERS!! NO ONE SHOULD BE ABLE TO ENJOY THINGS I DISLIKE!!1" with no real evidence of it being bad for the platform. Chances are, since it is still a popular and growing game, it will move more switch units, so, it is actually better for nintendo and the common switch user, since more units sold = more future third party support.
As of myself, i would fucking buy a switch right now if fortnite were on it, since playing on a mobile device without true controllers kind of sucks...