If a Jew wishes to visit a grave at a Christian cemetery (say the grave of a patrilineal grandparent), is it permissible? Does it matter if it is a mostly unadorned cemetery vs. one with pictures of Christian saints or other iconography?
yeshiva.co answers the question whether a Jew can attend a non-Jew's funeral
A Jew can enter a non Jewish cemetery and attend a non Jewish funeral. (see Bava Metziah 114a). The only prohibition is to enter a church if the ceremony takes place there.
According to Jewish law, there is generally no issue with attending a non-Jewish funeral or visiting a non-Jewish cemetery.
If there are pictures, one should not behave in any way that leads to concerns of avoda zara (e.g., prosternating).