Actually, Milotic walls Way better according to stats, but there are a few things Jellicent has over it.
Being Part Ghost Jellicent has STAB Ghost moves and Access to a...
Jellicent can run some sick nasty sets, Including the ever so loved - (Willo Wisp + Hex) - combo.
Even Though Milotic Can Take Water moves Like a Boss, a Slightly Injured Jellicent Can get 1/3 of his Health back from any Water Type move, which also heals off any Stealth Rocks damage you might have taken.
Okay, this is a biggy, Normal and Fighting moves don't even Touch him, Which makes him a Great Conkeldurr switch in, With Conkeldurr being a large threat in the OU Meta Game, Jellicent can take a Fighting move with Nothing happening, and Then Willo Wisp, Recover any Coverage damage taken and Hex his Face off.
So, As you might have seen, its not always all about stats.
My Reccomending Jellicent Moveset and Ev's Etc.
Jellicent (M) @ Leftovers
Trait: Water Absorb
EVs: 132 HP / 96 Def / 120 SAtk / 160 SDef
Modest Nature (+SAtk, -Atk)