Texting unknown users on whatsapp to be friends ,is messaging someone a crime ?

Yes, you have committed the offence of 'Stalking' against the girl.

Stalking is defined as a criminal offence under Section 354D of the Indian Penal Code (IPC). The section defines stalking as the act of following or contacting someone repeatedly, despite clear indications of disinterest or lack of consent, causing the person to feel alarmed or harassed.
According to the IPC, stalking can include online stalking which is repeatedly contacting a person online through social media, messaging apps, emails or other electronic means, despite their lack of consent.

Similarly, Section 67 of the Information Technology Act, 2000 (IT Act) makes the publication or transmission of sexually explicit content an offence. This can be applied in cases where a stalker sends sexually explicit messages, images, or videos to their victim.

Section 67A of the IT Act makes the publishing or transmitting of sexually explicit content through electronic means an offence. This section can be applied in cases of online sex chat, where the stalker sends sexually explicit messages or images to their victim.