As long as it is not malicious, it is not illegal to use the national flag as an avatar. If you want to use the national flag as the head of WeChat out of patriotism, you should pay attention to maintaining the integrity of the national flag, and don't scribble or deface the national flag, otherwise it will be illegal.
Legal basis:
Article 20 of the National Flag Law of People's Republic of China (PRC) stipulates: "The national flag and its patterns shall not be used as trademarks, patterns granted patent rights or commercial advertisements, and shall not be used in inappropriate occasions such as private funeral activities." There is no rule that it cannot be used as a WeChat avatar.
Article 23 of the National Flag Law of People's Republic of China (PRC) stipulates: "Whoever intentionally insults People's Republic of China (PRC) and the national flag by burning, damaging, scribbling, defacing or trampling in public places shall be investigated for criminal responsibility according to law; If the circumstances are minor, the public security organ shall impose a detention of not more than fifteen days. "