From an untested standpoint, Malady actually sounds like it synergizes well with Poppy. More attack speed means you're be hitting more often with your Paragon buff, and you'll probably hit three or so times between Devastating Blows, which are now stronger because Malady gives AP, to boot, and then there's the rest of your team that can benefit from the MR shredding if Poppy can't finish a target...
Note that the active on your W makes getting stacks for it by attacking less necessary than other champions with similar abilities.
I just think it's not worth an item slot compared to other stuff. I think it needs an upgrade. 25 AP is how much Sheen gives, and Sheen's passive is pretty much always better. And Sheen has two upgrades for when you need to have more than 25 AP and an early/midgame passive on an item.
Well yes as an AP item its pretty pathetic, but its not meant for building up AP. Tbh Malady is probably the most cost efficient item there is.
Malady costs 1825g, it gives 20 bonus magic damage for autoattacks (that's like 20 AD), 50% Attack Speed, 25 AP and shreds magic resistance by 6 per hit to a max of 24.
A Recurve Bow increases attack speed by 40% and costs 1050g, so for attack speed Malady gives something like 1300g worth, then the 25 AP makes up the rest seeing as amplifying tome costs 435g and gives you 20 AP, your pretty much getting 20AD and shredding magic resistance for free.
Malady is absolutely amazing for champs who use autoattacks to deal magic damage like Jax, Teemo, and Kog Maw. Malady + Mandred's on Kog destroys whatever comes into his attack range. The attack speed+ap+mag res shred+mag dmg all work well for a hybrid Poppy making her a killing machine!