Store aliases locally
The Tor Messenger is intended to help anonymize IM metadata. For contacts to remain anonymous, they must create accounts with usernames which are not associated with their known identities. This creates a usability problem because all of your contacts' usernames may be unrecognizable gibberish.
The natural response is to set an alias for your contacts so you can remember who they are. These aliases are stored server-side. This means that even if you do everything right, send messages only with both OTR and Tor, and pick a username that can't be traced back to you, your contacts could still easily and even accidentally reveal your identity to the server, which could be compromised or compelled to provide this data.
The Tor Messenger should add storage of contact aliases in the client and disable sending of those aliases to servers.