Depends on the creature.
Unlike 3E, in 5E there is no general rule on incorporealness -- each creature with an incorporeal-like ability has its own rules for how that functions. In most cases, it's simply reflected in the creatures damage resistances and immunities, and not called out as a unique ability, though most creatures that a 3E-era gamer would consider 'incorporeal undead' do have an Incorporeal Movement ability that allows them to pass through solid objects.
So for example, a ghost that is currently using its Etherealness ability can't be polymorphed by a caster that isn't also on the Ethereal plane. A specter, though, doesn't have that ability, so would be vulnerable.
Note, however, that when a polymorphed creature is reduced to 0 hit points, it reverts to its normal form, so polymorphing a spectre wouldn't let you kill the spectre without dealing with its damage resistance unless you use an effect that kills the polymorphed spectre without reducing it to 0 hit points.
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Pauper