if statement not executing?

Your Problem

Nothing appears to be wrong with the code if as you say is true, and the if statement on line 6 just isn't detecting the clothing.

So the problem isn't with your code, it's just one small overlooked detail. You are trying to detect a descendant of a model immediately after you initially detected the model itself. There is no time for the descendant to fully load! So it is nil when you check it on line 6.

How To Fix

To fix this, simply add a wait after line 3. This will yield the code and allow time for the objects to load.

Code

local ClothingRemover = coroutine.create(function()
    local OnChildSpawned = function(obj)
        print(obj:GetFullName())
        wait(1) --Yield
        if obj:IsA("Model") and obj:FindFirstChild("Humanoid") ~= nil and game.Players:FindFirstChild(obj.Name) ~= nil then
            for j,k in pairs(obj:GetChildren())do
                if k:IsA("Shirt") or k:IsA("Pants") or k:IsA("ShirtGraphic") then
                    k:Destroy()
                end
            end
        end
    end
    game.Workspace.ChildAdded:connect(OnChildSpawned)
end)

Goul's Efficiency Tips

I am unsure as to why all of the code is inside a coroutine.. but I'm positive that it is unneeded. You said in the chat that it was there because you have other code past this that needs to run at the same time.

Rather than supplementing for that fact by using a coroutine, I advise you make a whole new script in the workspace, that consists of a PlayerAdded event, and a CharacterAdded event with the same code you used to detect and eradicate all clothing.

--PlayerAdded event
game.Players.PlayerAdded:connect(function(plr)
    --CharacterAdded event
    plr.CharacterAdded:connect(function(char)
        --Same code!
        print(char.Name);
        --Yield until the torso is detected
        repeat wait() until char:FindFirstChild("Torso")
        for j,k in pairs(char:GetChildren())do
            if k:IsA("Shirt") or k:IsA("Pants") or k:IsA("ShirtGraphic") then
                k:Destroy()
            end
        end
    end)
end)