Can an administrator access the iCloud Drive of another user on MacOS?
The administrator can change your Mac password, login as you and so gain access to your iCloud Drive.  And also gain access to everything controlled by your username - for example, your Keychain.
The only mitigation is that the administrator can't change the password back, so you might become suspicious when your login fails.
Also, an administrator is able (without your login details) to look at any file on the computer - that would include any iCloud Drive files cached on your computer.
I don't know the law in your country, but where I am an employer is entitled to access anything on a computer owned by the employer (subject to any other agreement between employee and employer).
Advice: Don't keep personal information on an employer provided and controlled computer.