Mulligans require understanding what cards work together and what cards work against your opponent. I might consider keeping Kabal Lackey if I can also keep a powerful secret like explosive runes, although I would never keep Lackey against hero's with 1 damage dealing hero powers.
Mad Scientist, Duplicate, and Loatheb are the main additions from standard to Wild Secret mage. Loatheb is a fantastic card for aggro decks, stopped expensive board clears such as Dragonfire Potion, Flamestrike, Twisting Nether, and often Brawl/Dragon’s Fury. Mad Scientist is one of the most powerful cards in the game and should be in nearly every wild mage deck.
Ice block is a tech card for aggro matchups, and should be removed in favor of a 2nd duplicate when facing control decks.