Poppy isn't very effective when built with hybrid items.
AP Poppy doesn't really want AD stuff since she's built around extreme burst. AD Poppy doesn't want AP stuff since she's building for sustained damage. Trying to build for both will leave you weaker on the whole (having meh burst and meh sustained damage is not as good as excelling at one or the other).
AD Poppy does benefit pretty nicely from having the magic pen mastery though, because all the damage from her Q (inculding the autoattack that triggered it, sheen procs, etc) is converted into magic damage.