Is spamming yellow messages a crime?

If it is just spam, it is illegal and does not constitute a crime.

Article 68 of the Law on Public Security Administration Punishment: Whoever makes, transports, copies, sells or rents obscene books, periodicals, pictures, films, audio-visual products and other obscene articles or uses computer information networks, telephones and other communication tools to disseminate obscene information shall be detained for more than 10 and less than 15, and may be fined less than 3,000 yuan; If the circumstances are minor, they shall be detained for less than 5 days or fined for less than 500 yuan.