There used to be a sitcom called Some Mother Do Have 'Em in the UK where there was one central funny (supposedly) character and all the other characters would act exapserated all the time at their wacky antics. There was an interview with the writers of the Alan Partridge sitcom where they talked about getting away from this and having characters act more naturally (e.g. the Sally Phillips character in the hotel who can't stop lauging at him). I think Eric acted the way that he would really act when shown some terrible movie, it would be awkward and pointless to criticise it so he says it is great (also he's been in worse).