I'm pretty sure that was fully their intention - the characters are supposed to look like anime plastic dolls you can buy in shops(the hair is a dead giveaway, it's shaded and textured to look like plastic dolls, not hair). Coupled with 'realistic' environments you get something like TeamAmerica, anime styled.TheChef said:Cartoony graphics mixed with realistic graphics is odd but what's even more odd is combining realistic lighting all around. It literally looks like giant dolls are playing golf on a real course (I know the BG is not photoreal).
Personally I think it looks very cool, and it fits the personality of the series(which was never taking itself very seriously, thank goodness) perfectly.
Anything cartoony/celshaded looking for characters would be extremely out of place - unless environments were changed too - not to mention it would be a dramatic change of direction from existing games in the series.