Yes, this is a simile, and it is also an allusion. I have been asked this question several times by students when I taught The Outsiders, and none of them had a clue what the term meant. The "chessy cat" to which Ponyboy refers is a character created by Lewis Carroll in his fantasy classic Alice's Adventures in Wonderland (thus, the allusion). The Cheshire Cat is best known for its "mischievous grin"--like Two-Bit's--and the cat also had the ability to disappear at will; upon vanishing, the grin was the last part of its body to be seen. Carroll did not originate the idea of the grinning Cheshire Cat; it had been referenced at least 100 years prior to Carroll's writing Alice's Adventures in Wonderland.