C-h v org-indent-mode RET says (emphasis added):

If called interactively, enable Org-Indent mode if ARG is positive, and disable it if ARG is zero or negative. If called from Lisp, also enable the mode if ARG is omitted or nil, and toggle it if ARG is ‘toggle’; disable the mode otherwise.

You probably want to say (edited as per the OP's answer to avoid misleading future visitors):

((org-mode ((eval . (org-indent-mode -1)))))

in your .dir-locals.el file.

C-h v org-indent-mode RET says (emphasis added):

If called interactively, enable Org-Indent mode if ARG is positive, and disable it if ARG is zero or negative. If called from Lisp, also enable the mode if ARG is omitted or nil, and toggle it if ARG is ‘toggle’; disable the mode otherwise.

You probably want to say (edited as per the OP's answer to avoid misleading future visitors):

((org-mode . ((eval . (org-indent-mode -1)))))

in your .dir-locals.el file.

C-h v org-indent-mode RET says (emphasis added):

If called interactively, enable Org-Indent mode if ARG is positive, and disable it if ARG is zero or negative. If called from Lisp, also enable the mode if ARG is omitted or nil, and toggle it if ARG is ‘toggle’; disable the mode otherwise.

You probably want to say

((org-mode
   (org-indent-mode -1)))

in your .dir-locals.el file.

C-h v org-indent-mode RET says (emphasis added):

If called interactively, enable Org-Indent mode if ARG is positive, and disable it if ARG is zero or negative. If called from Lisp, also enable the mode if ARG is omitted or nil, and toggle it if ARG is ‘toggle’; disable the mode otherwise.

You probably want to say (edited as per the OP's answer to avoid misleading future visitors):

((org-mode ((eval . (org-indent-mode -1)))))

in your .dir-locals.el file.