Jolteon + Air Balloon = Sky battle contestant?
Nope,it wil not
For sky battles, you have to use Flying-type Pokémon, or Pokémon with Levitate. However, even not all of those are candidates with Pokémon such as Doduo and Pidgey being exempt. Mega Charizard X can still be used in these battles despite not having the qualifying factors after Mega Evolving.
"The Iron Ball and Air Balloon have no effect on eligibility."
Therefore even if you give your Jolteon an Air Balloon it will still not be eligible to compete in Sky Battles.
Hope I helped :)