While Duskull does have a higher base Def and SpD than Misdreavus, its awful base 20 HP means that, at least with the spread you're using, Duskull actually has considerably less bulk than a standard Misdreavus.
<@ebeast> !dmg Sawk 31 252 @Choice Band | Duskull 31 4/252 @Eviolite | Stone Edge
<CDXCIV> Sawk's Stone Edge vs Duskull (90 - 107 HP) Damage: 49.45% - 58.79%
<@ebeast> !dmg Sawk 31 252 @Choice Band | Misdreavus 31 252/252+ @Eviolite | Stone Edge
<CDXCIV> Sawk's Stone Edge vs Misdreavus (105 - 124 HP) Damage: 32.41% - 38.27%
<@ebeast> !dmg Gardevoir 31 252 @Life Orb | Duskull 31 4/252+ @Eviolite | Psychic
<CDXCIV> Gardevoir's Psychic vs Duskull (97 - 114 HP) Damage: 53.3% - 62.64%
<@ebeast> !dmg Gardevoir 31 252 @Life Orb | Misdreavus 31 252/4 @Eviolite | Psychic
<CDXCIV> Gardevoir's Psychic vs Misdreavus (142 - 169 HP) Damage: 43.83% - 52.16%
Misdreavus also has a much better speed tier with base 85 Speed that allows it to get off quick Taunts, Will-O-Wisps, and Pain Splits. The ability to get them quickly is great and Misdreavus still holds a decently powered Shadow Ball to do damage. The only thing Duskull really has over Missy would have to ironically be its low HP. With its low HP it can abuse Pain Split better, but in the end Misdreavus just does a better job at everything.