Assalaamu alaykum. In our area, the custom of limiting the total marriage celebration to the ceremony in the mosque in a very simple manner is called Sunnati Nikah. If I tell someone that I do not like this Sunnati Nikah and want to celebrate my marriage more gorgeously, will I be gulty of belittling the Sunnah of the Prophet, sallallaahu 'alayhi wa sallam?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

By the Sunni marriage, the people intend  the marriage that is in accordance with the rulings of the Sharia, meaning that it should not include what causes the Wrath of Allah, such as forbidden mixing, musical instruments, impudent singing, and so on. One should submit himself to such rulings and accept them.

Therefore, if you submit yourself to such rulings and accept them while you find them difficult to endure by nature, then there is no sin on you. This is not considered as belittling the rulings of the Sharia. Hating something by nature does not contradict accepting it in terms of the Sharia.

In Zaad Al-Maseer, Ibn Al-Jawzi when interpreting the saying of Allah (which means): {While indeed, a party among the believers were unwilling.} [Quran 8:5]; said, "There are two opinions: the first is that they do not want you to set out; the second is that they dislike to be prevented from the booty. This is the dislike by nature, due to the hardship of traveling and fighting, and not because of disliking the command of Allah.”

Moreover, Ibn Al-Qayyim said in Shifaa’ Al-‘Aleel:

“Something may be beloved and pleasing on the one hand, and hated on the other hand, such as drinking a beneficial medicine that is disagreeable: the patient is satisfied with it while he hates it; fasting on a very hot day: the fasting person is pleased with it while he hates it; and making jihaad against the enemy: Allah says (what means): {Fighting has been enjoined upon you while it is hateful to you. But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you.} [Quran 2:216]"

On the other hand, the Sharia permitted permissible entertainment in marriage and anything that makes one happy. Al-Bukhari reported that ‘Aa’ishah, may Allah be pleased with her, married off a woman to a man from the Ansaar, and the Prophet said to her, “O ‘Aa’ishah, you did not have any amusement? The Ansaar like amusement.”

Allah knows best.