Yes, you can raise DFL to the level (2012) of lowest domain controller OS. The operating systems of member servers does not have any impact.
To activate the newest domain features, all the domain controllers must be running the newest Windows Server operating system version in the domain. If this requirement is met, the administrator can raise the domain functional level.
To activate the newest forest-wide features, all the domain controllers in the forest must be running the Windows Server operating system version that corresponds to the desired forest functional level. Additionally, the current domain functional level must already be at the newest level. If these requirements are met, the administrator can raise the forest functional level.
https://learn.microsoft.com/en-us/troubleshoot/windows-server/identity/raise-active-directory-domain-forest-functional-levels#summary
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