Nope, Maru definitely was not mortal, and technically not even a cat, though he preferred to be one most of the time. Cats weren't monsters. Vampires were. He could be cute and soothing as a cat and make people smile. He could cheer them up. Look how much Madoka giggled when he did something, the way she hugged him and told him he was cute. Would she have been the same way if he was just hanging around in his human form? Probably not...

And it was very hard to get ear pets when he was human. It would look weird and feel even weirder. So wasn't it the strangest thing that as a cat it felt good? Maru even leaned into the Shiny Guys fingers, wanting him to rub just a little bit harder.