Bullet is a cat, and the poem is a quick, affectionate sketch of him. Each line gives us one trait, like a list of things the speaker has noticed while living alongside him: he gets along with other cats, he loves his sister Sweetie, he licks everything, and he has to sniff everything too. The last line adds a softer note, that he is terrified of new people.
The speaker clearly adores him. The opening, "Beautiful kitty kitty," sounds like the way people talk to pets, warm and a little silly. The short, plain lines feel like fond observations rather than a formal description, and the repetition of "everything" makes his curiosity funny and a bit obsessive. Because the poem ends on his shyness, the mood shifts from playful to tender. We see that this confident, sniffing, licking cat is also nervous, and that the speaker knows him well enough to love both sides of him.