Picasso wasn't Catalan. He was born in Málaga (Andalucía).
But anyway, why could someone think he had foreigner ancestors because of the 'ss'? The 'ss' is very common in Catalan/Valencian as James Brandon said (assassí --> killer assassin; carabassa --> pumpkin; essencial --> essential; assolellat --> sunny... there are hundreds).
The only thing that makes me doubt about the origin of the surname Picasso is the fact that words that end in 'o' are very typical in Castilian, but rare in Catalan/Valencian, and they use to exist because of some castilianization of Catalan and even more in Valencian Catalan. It might be a Castilianized Catalan surname or the opposite.
But anyway, why could someone think he had foreigner ancestors because of the 'ss'? The 'ss' is very common in Catalan/Valencian as James Brandon said (assassí --> killer assassin; carabassa --> pumpkin; essencial --> essential; assolellat --> sunny... there are hundreds).
The only thing that makes me doubt about the origin of the surname Picasso is the fact that words that end in 'o' are very typical in Castilian, but rare in Catalan/Valencian, and they use to exist because of some castilianization of Catalan and even more in Valencian Catalan. It might be a Castilianized Catalan surname or the opposite.