It's a classic case of OFHD Obsessive Foot Hugging Disorder PLus, he thinks King Julien is the awesomeness, and since common people grovel at the kings feet, Mort takes it up a level wiht a side of freaky and a serving of nuttyness.
Because it's always been the only part of King Julien, who he adores, that he can reach, so, eventually, he developed an obsession for that part in particular. Or, at least, that's what I presume is the reason. I hope this answer makes sense. :P
**Spoiler Alert** In the Netflix Original Series "All Hail King Julien" they provide a clue in the very first episode. Towards the end of the episode "King Me" around 19min 11s, while King Julien is saving his abducted lemurs from the Foosa, he saves Mort por using his feet to dance/kick away the hungry predators. This is where Mort's obsession begins. I wouldn't call it a fetish, even though they tease throughout the series that his affection towards Julien's feet is somewhat sexual, but it is an intense obsession--and one that according to Smart Mort is the one thing that all versions of Mort throughout the multiverse have in common. Anyway, watch the series because it is hysterical. I hope this respostas your pergunta accurately.