Um.. let me clarify. They CAN attack Mr. Mime if they have 2 or fewer Energy CARDS attached, but if they have 2 of fewer attached, then they do no damage. And if you have Mime Jr. under Mr. Mime then you prevent effects done to you as well. All of this ONLY APPLIES if they have 2 or fewer Energy CARDS attached.
So if say... a Torterra attacks Mr. Mime and the player with Torterra has a Sceptile GE out, Torterra only need 2 energy cards to attack with Frenzy Plant. Mr. Mime prevents that damage even though they have 4 energy attached, Mr. Mime is affected by the number of CARDS attached.
One more quick example, if say a Lucario DP attacks Mr Mime with Aura Sphere, Mime prevents the 40 damage to itself, but the Lucario can still deal the 20 to one of the bench because Mime only prevents damage/effects done to itself.