[Sekto Springs]

Fish.
They're evolved from birds, and many birds love to eat fish.
Abe's original story was that he was a fisherman living in solitude. His last name is "Lure" ffs.
Let's not forget the FMVs portraying Mudokons fishing in the Mongo.

I'd say fish and fruit, the latter being based on their habitat and Farzad's mouth design where he notes they are "healthy eaters compared to other inhabitants", and that their toothless maw accommodates softer, juicy foods. Since Abe and his mudokon buddies were given little teeth in Munch's Oddysee, it could be deduced that younger mudokons have an exclusively fruit diet whereas older mudokons are able to chew hardier foods like meat.