This kinda belongs to the Additive Fallacy category, where piling on more of something you already get automatically makes it a good idea.
Kogs 6% barrage for example. People used to love getting Bloodrazors because it "synergizes" with his barrage. It does promote getting lots of Attack Speed and have a certain anti-health playstyle. But its still just adding 4% for the razor...same as any other champ would have.
Malph already gets some built in Tiamat effect. Adding more isn't necessarily a great thing. The radius is really small and malph doesnt stack lots of AD making the Active effect quite weak.
malph as a bruiser does all magical damage from his Q,E,R. He has a minor steroid effect for his autoattacks...a dmg booster and the AoE effect. This isn't really enough to force him to discard all his magical damage in favor of AD and armor pen,etc.
ANYTHING will work, but I hope you can see that this is suboptimal. Either add some Mpen to optimize base damage values, more CDR for faster spam, or go tanky so you live long enough to tank well and initiate using your spells as utility mostly.