I wonder would a stereotype be more or less offensive if it had a basis in reality?
For example the Irish do have high rates of alcohol consumption and binge drinking. Although the states with the worst rates of alcohol dependence weren't Irish, there were a couples New England states noted for high Irish-American populations that were above average.
Current > Alcohol Consumption statistics - countries compared - NationMaster
Report Attitudes towards Alcohol
The Germans also had high rates of binge drinking and the heavily Germanic states of the Northern Plains are high in alcoholism going by Statemaster. There is a stereotype of Germans as drunks. So are stereotypes of drunken Germans or Irish (or American Indians perhaps) more offensive or less offensive? I actually might think it's more offensive because things are more hurtful if they have some basis to them however tenuous. If you asked a random Christian "do you know anyone who supports Hezbollah?" they might think "what the heck?" but if you ask it of a Muslim I think it's possible they'd be more offended. The vast majority of Muslims wouldn't know a Hezbollah supporter, but I think there would be some basis for thinking such people are bit more common with Muslims than Christians.
Anyway it's interesting that I don't think Americans have a stereotype on Scandinavians or WASPs as drunks. The British and Scandinavians generally have high rates of binge drinking as do the highly English or Scandinavian parts of the US. (Exempting Utah) Although in a Thanksgiving episode of
Grey's Anatomy Christina asks George "Meredith's a WASP, alcohol is like water to them so where is the booze?"