Tombs

The Rambam in Halachot, Chapter 4, Halachot 4 writes (something like this): “The righteous do not make for themselves souls (a building on the grave - Zion). Their words are their memory. And a person should not visit the graves.” Although according to the Radb, the explanation in the Rambam is that he only denies opening the grave, etc., but from the Ribb, in Tex. 22, it further implies that indeed, according to the Rambam, one should not visit the graves at all.
In any case, even for those who are permitted to go, there is a clear mishnah in the sign of Teknet, Section 1a, which says not to approach within four cubits of the grave for fear that they will stick to it, etc. And we will examine further in the first chapter 16: Regarding Hezekiah, who was seated on a seat over his grave, where Tosafot were written (4:5 who were seated), meaning at a distance of four cubits, then he mocked the king.

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I know of no prohibition against visiting a grave. Maimonides cannot create new prohibitions on his own accord.

Mocking Rash

A clear verse.

Whoever touches the dead body of any man who dies and does not repent, defiles the tabernacle of the LORD, and that person shall be cut off from Israel: because no uncleanness has been thrown upon him, he shall be unclean, his uncleanness is still in him. This is the law, a man, because he dies in a tent: everyone who comes into the tent and everyone who is in the tent shall be unclean seven days. 15 And every open vessel, on which there is no covering, is unclean. 16 And everyone who touches in the open field, one who is slain by the sword, or one who dies, or a human being, or a grave, shall be unclean seven days.