As long as it is not damaging or insulting the national flag, it is not illegal. Sticking the national flag on the upper arm, painting it on the face and putting it on the desk are all manifestations of respecting and caring for the national flag, which are all legal acts.
Legal basis: People's Republic of China (PRC) National Flag Law.
Article 19 No damaged, defaced, faded or substandard national flag shall be hoisted or used, and no national flag shall be hoisted or used upside down, inserted upside down or in other ways that are detrimental to the dignity of the national flag. Don't throw away the national flag at will. Damaged, defaced, faded or substandard national flags shall be recovered and disposed of in accordance with relevant state regulations. After the large-scale mass activities, the organizers of the activities shall take back or properly dispose of the national flag used at the event site.
Article 20 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.