Has there ever been a 6–5 in the Liga MX?

Yes, 3 times.

The first time: December 2, 1945, Asturias beat Monterrey (Asturias 6–5 Monterrey).

The last time: December 25, 1947, América beat Marte (Marte 5–6 América), 78 years ago.

Home wins: 2. Away wins: 1.

Nearby scores

1. 7–4 1 time
2. 7–5 2 times
3. 7–6 Never
4. 6–4 17 times
5. 6–5 3 times
6. 6–6 1 time
7. 5–4 24 times
8. 5–5 8 times

All games

1. December 25, 1947 Marte 5–6 América
2. November 9, 1947 Asturias 6–5 Atlas
3. December 2, 1945 Asturias 6–5 Monterrey