Maybe not. Steel is lighter than you might think. Bastard swords can be less than 5 pounds. But a war hammer of that size would be nearly impossible to use on horseback. Nerd out.
Robert was huge and insanely strong. It's mentioned that Ned could barely lift his hammer with both hands and Robert only used one with it. The actor in the show was great but they didn't really capture Robert's size. He's actually an inch shorter than the Hound, but much larger. Think Martin said he's 6'6.