Since Lemmy is not a single entity but a group of people hosting the servers, there is not official app.
Your question is similar to “There is no official email app?”.
There are hosts like Outlook and Gmail. Similarly, Lemmy.ml and Lemmy.world would be the equivalent. While outlook and Gmail have their own apps, individual instances rely on third party apps like Jerboa, Liftoff, etc. for mobile users.