Even “dev mode” itself is commonly used. For instance Atlassian used it in products that pre-date Figma’s copyright by years. And it’s a common feature name in countless open source software projects.
Figma tells TechCrunch that its trademark refers only to the shortcut “Dev Mode” – not the full term “developer mode.” Still, it’s a bit like trademarking the term “bug” to refer to “debugging.”