Definitely it is okay to do so, although I suggest not including personal information that you wouldn’t want other to read, such as the groundskeepers and other visitors.
In case you are not aware of a very widespread custom, I feel remiss in mentioning that there are traditional prayers and Psalms that are said when visiting a cemetery, which are found in conveniently small booklets that are readily available. Of course, one may say any personal prayers to God or words to the deceased, as desired.
In addition, before leaving the cemetery it is customary to leave a small rock, stone, or piece of grass, on the grave, in honor of the deceased, to show that one visited his/her grave. There is a tremendous feeling of comfort when going to a grave where there are such “reminders” left there, since it indicates that others have also been there. Another reason for using a stone: The Hebrew word for stone is "even" spelled aleph, beit , nun. This word can also be seen as a combination of the words for father "av" (spelled aleph, beit) and son or child "ben" (spelled beit, nun). Putting a stone there shows the continuity of the life of the deceased through his good deeds, family, and the afterlife.
