SpaceVim uses vim-airline which depends on a power line patched font. These fonts include extra symbols that render as arrows in the terminal.

You can find prepatched fonts here: https://github.com/powerline/fonts.
I’m pretty sure that repository also has instructions for patching an arbitrary font if none of the fonts it provides are to your liking. 

After installing one you’ll need to switch to using that font in iTerm2.