If Kauravas have won the battle, would the Mahabharat have been written in favour of Kauravas making Pandus the villain?

Going by the scriptural- read, real- account since we are not brain-addled modernist scholars like the chap Whatshisname who wrote Asura, the answer is…

No.

The Mahabharata was composed by an Ascetic, who'd come to Enlightenment and Fame long before the birth of the Kuru generations featured in the Mahabharata. While it was publicly recited at a gathering initially organized by the Kuru descendants, the recitation happened after the Sacrifices were (unsuccessfully) done by the same Ascetics who'd helped stop the Sacrifices and break up the Gathering. Ascetics wouldn't be Ascetics if they weren't Ascetics.

The themes of the Mahabharata are broadly Four-fold.

1- The inevitability of Fate and Man's Struggle against It.

2- The horrors of Civil Strife and the ruin of States.

3- The Nature of Aryahood and What it means to be one.

4- The Necessity of Dharma and how it must be upheld, no matter what the Cost is.

All this in the backdrop of the Collapse of the Kurus, the annihilation of most of their Empire- and that of their Vassal Kings and Allies, and the idea that despite all their might and glory- they'd failed to check the entry of Kali Yuga.

This is the Core lesson Hindu Traditionalists are to learn from the Mahabharata. Classical scholars and Brahmins however understood that the Masses are incapable of appreciating such nuance and thus diverted them into celebration of the heroics in the Epic.

Which turned out to be a bad idea because the Classical chaps forgot that they were in Kali Yuga and their ideas are corrupted from the start. Shortly after, the Masses started calling for Liberalism, Secularism, Freedoms, Feminism, Post-modernism, etc etc, garlanded Shri Rama with shoes, saying that Ma Sita should've slept around (a big Quora TW said this BTW), going around spitting on the Immortal Gods while worshipping random 7th Century Arab slavers, the Paedophile Periyar who was marrying teens in his 80s, and that Kashmiri chap who double-crossed Netaji.