Can we say that visiting the tomb of Egyptian kings is haraam?

The Prophet (ﷺ) said,

"Do not set out on a journey except for three Mosques i.e., Al-Masjid-AI-Haram, the Mosque of Allah's Messenger (ﷺ), and the Mosque of Al-Aqsa, (Mosque of Jerusalem)." [1]

Therefore, it is not permissible to make journeys to visit graves with the intention of ibadah. That includes to visit the Prophet’s  (ﷺ) grave in Masjid an-Nabawi, whereby one’s intention should be to pray in the masjid. While there, one can visit the Prophet's  (ﷺ) grave.

As for tombs of Egyptian kings Pharaoh, Allah has said in the Qur'an:

“So today We will save you in body that you may be to those who succeed you a sign. And indeed, many among the people, of Our signs, are heedless.” [2]

Based on this verse, it is not wrong to do so. Allah knows best.

References:

[1] [Sahih Bukhari Vol. 2, Book 21, Hadith 281]

[2] [Qur’an 10:92]