Yes, Mock data will be deleted and will be replaced with Tally data if you have integrated tally by following the steps below:
Navigate to Integration under Apps
Click on Tally
Enable two-way sync.
Click on Permissions and enable toggles for Invoices, Bills & Contacts, etc.
Click on Download the Tally plugin
Paste the Plugin TCP file in the Tally Source Folder
Complete the Config on Tally.