What is the purpose of aluminium foil on the wheels?
They are reflective covers to protect the tires from the elements, especially UV. Similar to the shades visible in the windscreen. Goodyear says:


  Aircraft tires, like other rubber products, are affected to some degree by sunlight and extremes of weather.
  While weather-checking does not impair performance, it can be reduced by protective covers. These covers
  (ideally with light color or aluminized surface to reflect sunlight) should be placed over tires when an
  aircraft is tied down outside.


It also protects the tires from other substances like oil and hydraulic fluid.

The plane is not necessarily mothballed. I would expect it to be parked much more out of the way in that case. But the wheel and window covers do suggest that they expect it to be parked there for some time.

Information is a bit sparse, but it appears that Happy Air only has that one Saab 340 now, but since September all flights are operated by Nok Air, who operates Dash 8 Q400, ATR 72-500, and 737-800 aircraft.