Since Renly didn’t talk about the incest did he give any excuse for his attempt to take the throne other than ”i will be a great king”.

Nope. Renly’s campaign pitch was: A. the glamour and magnificence of his person and his court, and B. the size of his battalions, while trying to get everyone to forget about the larger implications of any of it. 

As usual, Stannis puts it best: 

“Good men and true will fight for Joffrey, wrongly believing him the true king. A northman might even say the same of Robb Stark. But these lords who flocked to my brother’s banners knew him for a usurper. They turned their backs on their rightful king for no better reason than dreams of power and glory, and I have marked them for what they are.”