Was the religious and cultural diversity a core structural cause of the Ottoman Empire's decline?

it was ended by military defeats following the rise of the arabic nations near the Euphrates and the partitionning after WW1 and the laic reforms by the young turks who took power

There is no supply of reliable historical evidence indicating that cultural and religious diversity contributed to the decline of the Ottoman Empire. From its earliest beginnings, the major cities of the Ottoman Empire were some of the most ethnically and religiously diverse cities in the world, centuries before multicultural cities, such as New York City, Chicago, Los Angeles, Toronto or Buenos Aires.

Cities, such as Constantinople itself/(the name , "Istanbul", does not come into existence until the 1920's), Thessaloniki, Izmir/(Smyrna), Cairo and Alexandria in Northern Egypt, had various ethnic and religious communities during Ottoman times. Turks, Greeks, Armenians, Jews, Egyptian Muslims and Christians had lived in these major Ottoman colonial cities for centuries and there was little disruption or internal disquiet that is historically documented during the early-middle years of the Ottoman Empire.

What contributed to the decline of the Ottoman Empire.......was the Ottoman Empire. From the failed 1683 siege on Vienna, into the early 19th century, the Ottoman Empire, was imploding. It was becoming an increasingly anachronistic imperial state that consisted of widespread corruption, as well as less than competent Sultans. It was also a part of the world that, despite its centuries long presence in Europe, was far from being European-(culturally speaking). In other words, the Ottoman Empire, during this above mentioned time, when compared with much of the European continent, was culturally and intellectually unsophisticated and at a long cultural distance.

These are the characteristics that may provide a solid foundation for better understanding the root causes and origins of the Ottoman Empire's 200 plus year internal decline.

More a comment than a satisfying answer, but since I am lacking the reputation to comment, I have to do it this way: Santiago's answer is more related to the acceleration of the decline.

The defeat of the Ottoman army during the second siege of Vienna in 1683 marked the beginning of the downturn of the Ottoman Empire - more than 100 years before European Nationalism and Industrialization were about to happen.

The reasons for the decline are numerous, various and actually not that easy to detect. E.g. a rigid bureaucracy and government, ethnical and cultural diversity, a policy of aggressive expansion & suppression and the economic burden of a great army and lost battles combined with inner conflicts weakened the empire as well.

After 1683 the Ottoman rulers attempted to regain old power and influence - but due to a growing technological gap to Europe and the emergence of nationalism in the 19th century (causing uprisings in the Balkans) as well as European colonization this attempts failed.

The history of the Ottoman Empire is quite interesting - yet this topic is broadly neglected in studies of European History (although the Balkans belonged to the Empire for several centuries).

Religious and cultural diversity were part of the Ottoman Empire during its whole life, hence this alone can't be the reason of its decline. Also, the Empire reached its max size around 1680 and remained stable for a century or more.

What weakened the Ottoman Empire was:
- European industrialization
- Colonialism
- Nationalism

In short industrialization gave the european powers enough weapons and strength to defeat the Ottoman Empire. But also gave to european powers motives to search for new resources to supply their factories, new markets to sell their products and enough money to invest in other countries. This surplus gave to Europe the need to control foreign lands.
This is one of the reason that starts colonialism, were Europe starts to colonize several regions of Africa, Asia and Oceania. This turned Ottoman territory into a potencial place to colonize.
Finally, the surge of nationalism, one of the effects of French Revolution. This is somehow related to the question, because nationalism is related to cultural and religious differences. Nationalism gave a reason to european minorities (serbs, greeks, bulgars, romanians, etc.) inside the Ottoman Empire to fight for their autonomy.

Now, even though cultural and religious differences had an effect, the main reason was industrialization. Because the same effect ocurred in China, India or Persia, were european countries started to divide these countries in influence zones since they had the strength to do it. Only Japan was able to remain independent because they accepted western industrialization, and they were able to become a colonial power as well.