The Kalastajatorppa is my favourite hotel in Helsinki but it is definitely not for one that wants to be close to the city center or the action!
The only practical way to get to the "Torppa" from the airport is by taxi. I think it will be about 50€ each way (haven't overnighted nor taken a taxi in Helsinki for a long time).
However, once you are there, there is a convenient tram connection to the city center. Tram nr. 4 takes about 30 mins max.