It depends what you mean by myths.
If you mean, are they true? Then some things may be—there probably was a king in Mithila called Janaka—while others obviously are not—animals don’t talk.
If you mean “sacred stories” then they are stories passed down as sacred scripture, so that makes them myths.
However, in what I consider to be the “true” sense of myth only a few Jatakas would qualify. Most Jatakas are simple moral fables, more comparable to Aesop or fairy tales than to deep myth.
My own view—which is admittedly a highly personal one—is that true myth hails from before the axial age, and is concerned with the shift in consciousness of that time, depicted primarily as the creation of the world, or to put it another way, the death of god and the arising of humanity. Such myths include Gilgamesh, the Bible, or the Ramayana.
This cycle of deep myth has a number of highly characteristic features, including such motifs as the sacrifice of the sacred king. There are a fair number of such stories in the Jatakas, and I analysed several of them in White Bones Red Rot Black Snakes.
One example of this story type, relevant to the animal theme, is the set of stories dealing with the golden beast—a deer or peacock, who is depicted not just as a talking animal, but as a divine being, playing a crucial role in the destiny of not only his people, but the world at large. Here we could also include the Chaddanta Jataka, the magnificent tragedy of the white six-tusked elephant. In such stories, the divine animal, through their wisdom and sacrifice, creates a covenant of peace and harmony that reconciles the forces of nature and leads to bounty and prosperity. But this can only happen through sacrifice. The age of the divine protector whose personal glory can safeguard his people is over, and what is needed is a legal contract that defines relationships and establishes boundaries of mutual respect.
See above: in some cases, yes. But the Jatakas are far more extensive, varied, and in many cases, older.