Can you tap artifacts to an improvise cost to help pay for kicker costs?
Yes, you improvise to pay the total costs of a spell and kicker is an additional cost.

This is per Comprehensive Rules:


  702.125a Improvise is a static ability that functions while the spell with improvise is on the stack. “Improvise” means “For each generic mana in this spell’s total cost, you may tap an untapped artifact you control rather than pay that mana.”


Total cost is defined in the Casting Spells rules:


  601.2f The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. (...)


And kicker is an additional cost:


  702.32a Kicker is a static ability that functions while the spell with kicker is on the stack. “Kicker [cost]” means “You may pay an additional [cost] as you cast this spell.”