I'd say it could be called either way...
Pro: Telling Blow is a feat allowing you to trigger your skirmish or sneak attack damage by adding another trigger criterion (making a critical hit) to the usual (movement/opponent flanked or denied Dex). Therefore you'd do as much extra damage as you could with any skirmish or sneak attack, including Improved Skirmish.
Con: Despite the name, the Improved Skirmish feat does not improve your "skirmish" ability: if it did, it would increase your damage on any and every skirmish attack. It in fact grants you - to all intents and purposes - an
extra ability called "Improved Skirmish", which is triggered by moving 20' before attacking and which adds to your the bonuses which you get from making a skirmish attack, which is in turn triggered by moving at least 10'. It looks like much the same thing, but only because it's impossible to meet the criterion for an Improved Skirmish attack (20'+ of movement) without also meeting the criterion for a skirmish attack (10'+ of movement)
Therefore you don't get to add the extra damage from an Improved Skirmish when invoking Telling Blow. All Telling Blow does is add a trigger criterion for skirmish damage ("making a critical hit" as well as "10' or more of movement before attacking") but you've neither implicitly nor explictly met the criterion for an
Improved skirmish (20' or more of movement), nor is your critical hit an actual skirmish attack (as illustrated by the fact that you don't gain any other benefits of skirmishing, such as improved AC). In much the same way, you wouldn't be able to invoke any special abilities triggered by making a sneak attack (such as a Razing Strike) because you haven't met the criteria for making a sneak attack, only for "doing extra damage as though you had made a sneak attack".
Just thought I'd play Devil's Advocate
