The two logos aren't identical, but they share the same principles
Microsoft's website that hosts the description about the SQL Server Express uses a logo for the community that is extremely similar to the one used and trademarked by Canonical for Ubuntu.The differences between the two designs are significant when placed next to one another, but the same drawing principles and ideas were used to mark the fact that it's all about community.
The logo is probably not similar enough for Canonical to send a trademark infringement letter to Microsoft, but it would be interesting to know how that would work between the legal teams.
The logo on Microsoft's website is also accompanied by a text: “Join the SQL Server Express Community to access resources, connect with peers, and learn from Microsoft experts.”
You can check the similarities for yourself on the official website.