Despite LTR languages, write.as uses user system default font for RTL languages.
I suggest using Vazir (Serif), Shabnam (Sans-Serif), and Vazir-code (Monospace) fonts. They’re open-source fonts that are already used as default in Plume, Mastodon, OpenStreetMap and Telegram.
In the meantime, you can load them up into Write.as or WriteFreely via Custom CSS to use them for your blog. There’s a forum post that highlights how to do this here: