How good would a masterwork platinum toy hammer be as a weapon?
The platinum toy hammer would be OK as a weapon, but I am pretty sure masterwork quality would do nothing for you (I think that only applies to attacks with weapons) and I'm not sure if it's small size would result in a contact area as small as whips, warhammers, or arrows. Even if it did, contact areas below 10 (warhammers have this) are all pretty much the same for armor penetration due to rounding that occurs.
The reason whips have superior armor penetration compared to warhammers is not because they have contact area 1, but because they have a 5x velocity multiplier compared to 2x for warhammers. Damage is directly proportional to weapon momentum. Momentum is of course mass times velocity, so increasing the velocity increases damage (and armor penetration) by the same amount. Increasing mass will result in increased momentum, but the velocity of the swing also decreases with mass, resulting in overall momentum increasing somewhat, but less than linearly, with mass (Urist da Vinci has a post with exact numbers somewhere). Anyhow, your platinum toy hammer will get a velocity multiplier of 1x, making it significantly less effective than a whip or warhammer. If you could get an artifact platinum whip or warhammer, now that would be truly fearsome (although ordinary whips are already ridiculous against humanoids, so I'm not sure how much better a platinum one would really be).
TL:DR - don't bother trying to make a platinum toy hammer, unless you just want to do it for the lols.