Is there an error in a Wikipedia article explaining the influence of oxidation states?
Please do not edit the Wikipedia article, as your wording is incorrect, and the existing wording, while awkward, is correct.
What the article is trying to say is that the electron density on each oxygen is less as the oxidation state of the Cl increases. We can understand this by imagining progressive oxidation starting with $\ce{HClO}$. Here, the Cl has oxidation state +1 and the O is -2. But of course the oxygen doesn't have a full negative 2 charge. Some of that charge is shared with the chlorine and the hydrogen, neither of which has a full charge of +1.
To get to $\ce{HClO2}$, we can imagine reacting $\ce{HClO}$ with a neutral oxygen atom (oxidation state 0). After the reaction, the oxygen will have oxidation state -2, and the Cl will now be +3. Again, these are not the formal charges, but give us a hint about where charge will be located. The oxygen will have a partial negative charge, which means the chlorine will have to lose some of its electron density, increasing its actual charge. The original oxygen from $\ce{HClO}$ is still there and feels a stronger &quot;pull&quot; on its electron density, so it gives more to the chlorine, and the oxygen's partial negative charge becomes less negative.
Completely making numbers up, we could imagine that in $\ce{HClO}$, the charge on oxygen is -1, on chlorine is +0.25 and on H is +0.75. After adding the second oxygen to make $\ce{HClO2}$, the charge on the new oxygen might be -0.5, on the already attached O might be -0.75, +0.75 on H and +0.5 on Cl. Do you see how the negative charge on each O gets smaller in magnitude, but the total negative charge on oxygen(s) gets larger in order to offset the increased positive charge of the more oxidized chlorine? The Wikipedia statement refers to the individual O's not the sum.