Agreed that it could be a nice option but it's already possible now using preloaded css:
add an import statement and then apply that font to your retool canvas element - I've used it lots, and with icons too
@import url('https://fonts.googleapis.com/css2?family=Mulish:wght@600&display=swap" rel="stylesheet">');
@import url('https://fonts.googleapis.com/icon?family=Material+Icons|Material+Icons+Round');
.retool-canvas h1,h2,h3,h4,h5 { font-family: 'Mulish' !important; font-weight: 600 !important; }