Reupload from old forum. Thanks @Senk-Ju for the tutorial.
LiquidBounce allows you to use custom fonts inside the HUD. Both TTF and OTF fonts are supported. Just follow the instructions below to get started.
1.) Download a font from the internet (Example: https://www.dafont.com/de/).
2.) Place the font file(s) inside LiquidBounce's fonts folder (default Windows path:%appdata%/.minecraft/LiquidBounce-1.8/fonts).
3.) Open 'fonts.json' inside a text editor of your choice.
4.) Paste the following code inside the json file, replace the value of 'fontFile' with the filename of the font you want to add and save it.

[
    {
        "fontFile": "font1.ttf",
        "fontSize": "40"
    },
    {
        "fontFile": "font2.ttf",
        "fontSize": "50"
    },
    etc...
]

Example:

[
    {
        "fontFile": "montserrat-regular.ttf",
        "fontSize": "40"
    },
    {
        "fontFile": "roboto-regular.ttf",
        "fontSize": "50"
    }
]

5.) Run LiquidBounce and open up the CustomHUD screen by pressing the blue button in the bottom left corner of the screen.
6.) Select a CustomHUD element and change the font to the one you just added. To change the scale of an element, use your mouse wheel while hovering it.

The process is the same for every CustomHUD element.