This is caused by corrosion from driving on wet salty roads in our typical UK winters. If you look at other 500's (and Pandas) of similar age, you'll likely find they are the same. There's not much you can do to prevent this happening.
It's unlikely to have been caused directly by the fitting of the radiator, but doing so may have loosened any fins already weakened by corrosion. This isn't a criticism of the mechanic doing the job, just an inevitable consequence of working close to a component with rotting fins.
The loss of fins will reduce the cooling capacity of the condenser a little, but probably not enough to stop the A/C operating satisfactorily. Eventually, the corrosion will perforate one of the main refrigerant pipes in the condensor, and then it will either have to be replaced, or you could instead just run it as a car without A/C.
Until then, it's probably best left alone. It might even last the life of the car in its current condition; the fins are much weaker than the main pipework.
Even if you replace it, its replacement will likely go the same way in a relatively short time. Another example of Fiat parts quality.
Anyone suffering a perforated radiator due to external corrosion (as opposed to internal corrosion caused by running without the correctly inhibited coolant) will likely find their A/C condenser is in a similar state. They're made of the same materials, and located in much the same place, so equally vulnerable to the effects of salt spray.