Not at all, people in Kerala are used to receiving a lot of European and American tourist who usually wear shorts and sleeveless, so wear shorts and sleeveless or for that matter anything which makes you comfortable.
Loose-fitting slacks and a loose fitting shirt will provide airiness and protect against too much sun exposure.
As a tourist she will be permitted a great deal of latitude. But as a young women in India she will be subjected to altogether too many unwanted advances even without dressing in a manner that conservative Keralites might find provocative. It will be worse if she is a tourist, worse yet if she is unusually attractive to begin with.