I use FiraCode Nerd Font Mono instead, but it also does not have a specific italic font. In my config, only the “normal” section is defined, but all my bold and/or italic text looks like it should. Apparently, alacritty will apply a heavier weight or slant to the “normal” type face if you simply omit the “bold” or “italic” sections. So, what you have right now should Just Work.
Allegedly, you can omit the “style” specification in the “italic” section (ie: just add “italic”: {“family”: “FiraMono Nerd Font”}, to your config snippit above), but i haven’t actually tried doing it that way.