To be fair, just about any champion's build will differ if they get fed.
However, Morde's ratios are pretty terrible as far as burst mages go. They have to be, because they're AoE. His Q has a .4 ratio if the target isn't isolated. (It actually scales better with AD since it's an auto attack modifier, giving it a 1.0 AD ratio) His E has a very modest .6, and his W has a .9 if you stand next to someone for the entire duration. (Good luck with that on a 0 mobility champion like Mordy)
Those ratios are a far cry from other burst mages. His base damages, on the other hand, rival that of tanks. They're very high, to the point where you would need 416 AP just to have the scaling match the base damage on his rank 5 E. His Q is even worse, requiring 500 AP to tie the base damage you get just for maxing it out if the target isn't isolated.
While building AP isn't bad, Mpen will actually give you more damage in the early-mid game for less gold cost against most enemy champions. In addition, being a melee mage like Morde means that you need more than just pure damage, you need to be able to take a hit. Mpen items like
give you tank stats, a large boost to damage, and for much cheaper than some combination of
or
. Those items do have their place on Morde, but they're situational and it wouldn't be prudent to rush them.