Quite easily actually.
When we watch two people of equal talent and ability fight in real life we aware of certain realities of the fighter. If the fighter's mentality is not in it, if the fighter is not aggressive enough, if he's too calm, if he's not relaxed enough, it can cause major, major issues for the fighter and lead to him getting demolished.
Gohan literally does not want to fight. He is putting none of the effort forward required to do so. The only time he does so is when he gets mad and drops Cell on his butt. To me, that's the only time Gohan shows his true self as a Super Saiyan and what he is capable of against an opponent of equal value.
If that somewhat angry Gohan continued to fight Cell, it would be a good battle. Gohan simply does not have the stomach for battle. He is actually trying to negotiate his way out the fight. It isn't until enraged Gohan comes out do we see a warrior. He has the power, but not the heart Goku has, which is exactly what Piccolo talks about.