Praise be to Allah, and may peace and blessings be upon the Messenger of Allah.
It is authentically established in the Sunnah that Allah Almighty has forbidden the earth from consuming the bodies of the Prophets. The Prophet ﷺ said:
"Indeed, Allah, the Almighty, has forbidden the earth from consuming the bodies of the Prophets."
As for the bodies of people other than the Prophets remaining without decomposing, whether they are martyrs, righteous people, or non-Muslims, there is no evidence in the Quran or Sunnah establishing that this is a sign of righteousness or holiness.
The failure of some bodies to decompose may be due to natural factors, such as the nature of the soil, bacteria, and the materials surrounding the body. It may also result from the use of preservatives or methods that make the body appear as though it has not decomposed.
If the body of a righteous Muslim is found without having decomposed, we hope that this is a manifestation of Allah's mercy and favor upon that person. However, we cannot assert that it is evidence of their righteousness or an honor granted to them, because there is no religious evidence establishing this.
The same applies to the bodies of non-Muslims. Their failure to decompose is not evidence that their religion is true or false.
Conclusion:
The failure of a body to decompose after death is not a religiously established proof of the deceased person's righteousness or holiness. The exception confirmed by authentic texts concerns the bodies of the Prophets. As for everyone else, the matter requires scientific investigation, and religious judgments should not be based upon it.
And Allah knows best