Mark Addy is far from perfect. Robert was a demigod amongst men. Tall, strong, handsome, etc. Yes he went to seed, yes he was fat and well past his prime, but in no world was Mark Addy's Robert ever the Robert Baratheon who won the war swinging his enormous warhammer one handed and charming every girl who crossed his path. Likewise, Renly suffered for being just as unimpressive. I never believed for a moment that anyone would follow either of them into battle.
I haven't seen Renly on screen yet (still just seen the first episode!) but in the book he's described as the most handsome man Sansa had ever seen, and looking at the pics of the actor online... I guess she hadn't seen that many men.