"마음만이라도 고마워" The context is not given, but it probably means someone offered to help or verbally expressed support. Based on that, we might translate it like these: - I appreciate your moral support. - I appreciate your wanting to help. - I appreciate your offer whatever happens. (i.e, whether the offer to help materializes or not)

i think : thanks for your mind too !!! or thanks for even your mind/worry