Adding nicer fonts to your terminal vim is not as easy as one would think but here is a good path to follow to make it happen on a mac

brew tap caskroom/fonts
brew cask install font-hack-nerd-font

On your .vimrc

set encoding=utf8
let g:airline_powerline_fonts = 1

And then change it on iTerm2

This has been shamelessly ripped off from here

Now look how beautiful it looks with airline!