That's an ambiguous statement, depending on what exactly you're referring to by "DOS".
If by "DOS" you're specifically referring to shell (COMMAND.COM), then yes, it didn't know or care about the mouse. But MS introduced DOSSHELL (in '88), which had mouse support (along with other later core applications such as EDIT.COM), and of course, there were other thirdparty shells too (like Norton Commander) which also had mouse support.
But if by "DOS" you're specifically referring to the kernel (MSDOS.SYS), then you may be surprised to know that even the Windows kernel (NTOSKRNL.EXE) doesn't know or care about the mouse - this is handled by other bits like mouclass.sys and win32k.sys.
