Is rathi is a Punjabi khatri caste?

No. Rathi/Rathee belongs to Jat, Gujjars or Rajputs originally found in Haryana, Rajasthan, UP, MP and Gujrat. "Rathee gotra originated from province named Ratha (राठ), a republic of Ratha (राठ) Kshatriyas in south Gujarat and Rajasthan." (Wikipedia)