Actually, old Morde was way better. Higher base stats, more overrall damage, more rage (small, but still true), to and tankier.
His current damage is all put into his Q, making the rest of his skills weak. Pre-rework Morde had crazy high damage on his Q, and E, and was still able to retain decent damage on his W, all while being able to use these skills solo.
Old Morde had higher base health, health regen, armor, attack speed, and much higher movement speed.
Old Morde had higher range on his Q (~600 range from the AoE), slightly higher range on his E, and much more range on his R. Also, all of his damage is focused on his Q now, an auto attack. Before, his main damage was from his E, so he could at least poke at range.
Statistics may say he's "stronger", but that's just banrate, which is about 50% right now. His win rate is actually lower than pre-rework Morde!
Old Morde had about a 51% winrate, new Morde has a 49% winrate.