Mamma li turchi!
In the end of 1700, a group of Ottomans arrived in Sicily and during a lunch at Palazzo Comitini, a Turkish admiral was caught beating a girl, so everyone, indignant, started a real "Turkish hunt" and the kids ran down the streets shouting "Mamma li turchi!" that means "Mom the Turks!" as "Mom I've found a Turkish!"
Fumare come un turco
Turkish people during Ottoman Empire were associated with arab habits, one of this was the use to smoke shisha/narghile/hooka/water pipe.
Bestemmia come un turco
Puzza come un turco
These last two sentence has no historical meaning, they are just made associating concepts. "Bestemmia come un turco" may be the association of Turkish/Arab language that may hear rough to an Italian and may give the idea of someone cursing ("bestemmiare" means "to curse to God"). The last one is actually racist and I've never heard/seen it, a better and not racist way to say the same concept is "Puzza come un maiale/cane/capra" ("He/She smells like a pig/dog/goat").
Another example of association of two concepts may be "Mi capisci o parlo cinese?" that means "Do you understand me or do I speak Chinese?"
EDIT: As suggestion from DaG it can be assumed that the sentence "Bestemmiare come un turco" can be interpreted as a memory of the ancient fights against Arabs were Christians and Muslims fought against each other. The sentence so could be intepreted as the Turkey inhabitants cursed to God to evidence the fact their don't trust in Christian God and so they curses would be more evidenced. And I quote from the user:
But in this particular case, I believe it is safer to assume Turks where put together with every other “infidel” people who blasphemed in the sense of saying things contrary to the Christian God. If it were just a matter of harsh sounds, to an Italian ear, Germans, Russians or even Greeks might be sound as cursing too.