I have a 1949 Jefferson nickel, and the mint mark D is on the back to the right of Montecello, shouldnt it be on the front under the date?

From the years 1938 to 1964 the mint mark was on the reverse on the far right next to Monticello. This is the rule with an exception made for wartime nickels minted from 1942-1945. Wartime Nickels mint mark is on the reverse top center, only wartime nickels have an actual P mark. After 1964 the mint mark was moved to the obverse (front) of the Nickel. Source: https://coinbin.com/info/jefferson-nickel/

The same coin you are speaking of taken from PCGS website source : https://www.pcgs.com/coinfacts/coin/1949-d-5c-fs/84038

Hope this helps thanks for posting!