Hi there, it is but may not be too severe if it is taken off at this point. Sometimes the bad portion will just fall off + there will be no further damage but other times it keeps spreading upwards. You might want to have an experienced reptile vet look at it + possibly amputate about 1/4 inch above the bad area. [ Don't go to just an " exotics " vet who may know almost nothing about lizards unless you have no true reptile vet, and then ask what experience they have and look at reviews. ] For now mix some betadine [ drugstores carry it ] with water and dab it on the tail several times a day.
Do you say there is a bite further up ?