Isn’t the m9 bayonet a war crime because it is serrated?

Not explicitly on an international level, although many countries have themselves made various interpretations of widely-accepted general international conventions on the use of weapons in warfare and have published their own regulations on what types of weapons they believe to cause indiscriminate damage or unnecessary suffering and are hence prohibited for use by their own militaries (i.e., chemical or biological weapons, or weapons disguised as innocuous objects such as toys). Subparagraph "e" of Article 23 of the 1899 Hague Regulations concerning the Laws and Customs of War on Land states that “[I]t is especially prohibited...[t]o employ arms, projectiles, or material of a nature to cause superfluous injury.” The same subparagraph in the amended 1907 Regulations states “[I]t is especially forbidden...[t]o employ arms, projectiles, or material calculated to cause unnecessary suffering.” The Netherlands is the only country that specifically prohibits its military from using serrated or saw-toothed bayonets;

A bayonet or combat knife is not prohibited. It is prohibited to modify bayonets and knives, for example with a saw blade or barb. This prohibition stems from the ban on using weapons, projectiles or substances which may cause unnecessary suffering.

Sources:

Customary International Humanitarian Law. Edited by Jean-Marie Henckaerts, Louise Doswald-Beck, and Carolin Alvermann. Cambridge: Cambridge University Press, 2005.

Weapons That May Cause Unnecessary Suffering or Have Indiscriminate Effects. Geneva: International Committee of the Red Cross, 1973.

This is a common myth but there's no international convention that explicitly outlines serrated weapons as being illegal for use in war. The historic objects to the use of serrated blades during the first world war typically sighted part of the the 1907 Hague Convnetional VI.II.I.23 which simply reads

In addition to the prohibitions provided by special Conventions, it is especially forbidden - ….. To employ arms, projectiles, or material calculated to cause unnecessary suffering;

The argument has always been a relatively weak one and it is now largely agreed that as bayonets have become predominantly a utility tool having a serrated edge does not constitute material calculated to cause unnecessary suffering as the serrated section is designed to improve its ability as a tool not to cause unnecessarily bloody injuries.