One of our club members have flat fenders up front and stock fenders in the back of his JKU and I think it looks stupid.
Guy down the street from me used to have his 2 door set up this way. Don't know that I thought it looked stupid, but definitely a little strange. I just assumed he was building as he could afford to. I guess it was a good assumption because now he has flatties all around.
I don't think it looks too bad. I don't particularly care for flat rears either but have them because my tires rubbed on the stock front and rear fenders when my swaybar was disconnected after installing aftermarket wheels.