Yes, FigJam (and Figma overall) are very secure.
This is because Figma's security measures exceed industry standards for data protection and security.
Figma encrypts both data at rest and in transit. Also, all their servers live within Figma's own VPCs to prevent unauthorized network requests.
All data provided, and any content created, is owned by those customers.
Figma believes that it is their responsibility to protect your information and content.
Mentioned in this answer
FigJam by Figma
An online whiteboard from Figma designed for teams