Yes, you can play Terraria online on Android, but only as a local multiplayer solution through WiFi. There is no online with remote servers support like there is on PC.
MULTIPLAYER -- Local WiFi games support up to 4 players cross-platform on any mobile device! - Description page on Google Play.
As you can see below, the developer created this feature only for the WiFi devices.
If you need additional questions, use the following email to contact the developers: