If someone has access to your computer to access your clipboard history, you're already been pwned and the clipboard with random scattered entries is the least of your worries.
Sometimes you have broken websites/apps so you gotta copypaste. Sometimes they even have fields where you can't paste either (K9mail on android) (I cry in 64 char password).
It'd be an interesting feature for a password manager to issue a system call to purge clipboard history on copying a password. Lots of password managers aren't just browser add-ons but full desktop apps