Hi Ori, as far as I am aware there is not problem with frost forming on the top surface of the wing during flight. Extended flight at high Flight levels can cause the fuel in the outer tanks to cool to Jet A1 limits, larger Airbus allow for the transfer of fuel from outer to inner tanks, options then are to increase Mach and or reduce flight level.
You do however find frost forming on the underside of the wings under the fuel tanks after flight with cold soaked fuel. If the transit is long enough you will also get the formation of clear ice on the top side of the fuel tanks dependent on atmospheric conditions. There are limitations for the frost underneath and no ice is allowed on the top. Avoided by reduced fuel load on transit and by uploading warmer fuel. Ultimately de-ice prior to departure.
hope that helps.