I thought of John Prine but his stuff always has a bit of irony. A little bit sweet and sour. Jim Croce and Tom Paxton have a mix of happy and otherwise. Neil Young on the other hand, in his 500 songs, has written few happy ones.
Remember the Lovin' Spoonful? Mungo Jerry, "In the Summertime?" Happy Happy stuff!
Sugar Ray and Foo Fighters have done some happy stuff.