I think the idea behind 'Fall in Love One Nap at a Time' in the context of Malec fanfiction is quite creative. Naps can be a symbol of vulnerability and trust. In a relationship, especially for Magnus and Alec, it could mean that they are letting their guards down. It might show how they are growing closer in a very natural and unforced way. Maybe in the fanfiction, they start off just napping near each other and as time goes on, these naps become a special part of their relationship.
I haven't actually read this specific 'Malec' fanfiction. But generally, 'Malec' (Magnus and Alec from 'Shadowhunters') fanfictions are really popular. They often explore their relationship in more depth than the show sometimes can. Maybe this one focuses on the little moments between them, like during naps, which could be really cute and endearing.