Press Ctr + Shift + j to open devtools console in crostini terminal.
Paste this to use Jetbrais Mono with ligatures:
term_.prefs_.set('font-family', 'JetBrains Mono Nerd Font, monospace');
term_.prefs_.set('user-css-text', '@font-face {font-family: "JetBrains Mono Nerd Font"; src: url("https://raw.githubusercontent.com/ryanoasis/nerd-fonts/master/patched-fonts/JetBrainsMono/Ligatures/Regular/JetBrainsMonoNerdFont-Regular.ttf"); font-weight: normal; font-style: normal;} x-row {text-rendering: optimizeLegibility;font-variant-ligatures: normal;}')