Most fatbikes come with very knobby tires. tire friction is 100X more important than weight on pavement.
if you have a decent fatbike it only is maybe 5 lb heavier than an FS MTB, maybe even same weight if it is a cheap MTB. Compare to your own weight and you see unless you weigh 110 lb, it doesn't' matter much.
Obviously lighter is always better., you decide how much you are willing to pay for a certain weight reduction. If you are a weight weenie, look at the Canyon Dude.