Yes, there are some fantastic digital art apps for mobile phones. For example Procreate Pocket for iPhone is a paired back version of Procreate for iPad while Android has versions of ArtRage and Clip Studio Paint as well as Adobe Fresco and Infinite Painter.
You can now get small drawing tablets to connect to Android phones and iPhones too, and many of the main drawing tablet brands feature the right connections to paint and draw on the go. Two of my favourites are the Parblo Intangbo and the Wacom One.