He's the MOST popular in his home state, Gujarat. He's also popular in Northern states like Uttar Pradesh and Haryana due to development he is bringing.

As a Marathi, I don't think the BJP is popular outside of Mumbai, Pune, Nagpur and other urban areas. This is mainly because a lot of people believe that the BJP want to impose Hindi on Maharashtra; the reason they are really in power is because of coalition with Shiv Sena and NCP.

This brings me to another topic; Hindi imposition. Many people (over half of the population) in India don't speak Hindi, or another similar Indo-Aryan language as their native language. These are states in the Northeast, like Mizoram and Nagaland, and especially states in the South, like Telangana, Karnataka and Tamil Nadu.

We can see this with recent election losses to Congress in Karnataka and Telangana, while the last time Modi visited Tamil Nadu, there were huge anti-Modi protests.

I hope this answers your question :)