Sri Lanka is indeed a developing country. The confusion is, though, that it is said to have been developing for 70 years or more, ever since it won its independence. Just how long does it take a country to develop? That's the real question. Development of a country is generally 20 years or less.
It is considered a Lower Middle-Income country and it has an official GDP per capita of USD 4,073 or so it was in 2017. The total population is 21.4 million people The country is, however, making significant progress in the area of socio-economic development and in and human development too so you have to give them credit for that.