Is screen mirroring with Ditto secure?

Yes. Presenting device screens with Ditto is secure. Ditto screen mirroring is a point-to-point process. Screen data remains on your local servers and never crosses the internet. Ditto communicates with servers via industry-standard encrypted SSL technology. 

Additionally, Ditto does not store or access end-user information. 

Check out the links below:  

Other important security notes.

Data Flow Security Guide