Is the -ino suffix a diminution?

Family names are family names, and their suffixes (or other components) don't convey any special meaning right now; if anything, they did so centuries ago, when they came to be. It's very rare in modern Italian to modify a present-day family name.

^{In the special case of your example, by the way, “Sorrento” is (also) a city name, while “sorrentino” means “inhabitant of Sorrento”. So it's likely that the ancestors of someone named “Sorrentino” hailed from Sorrento. No diminutive form here.}

For first names and other nouns, “-ino” denotes in general, as you have gathered, a smaller, younger or even cuter version of the main form. “Tommasino” could be the way a “Tommaso” is called by his friends, especially if he is shorter or younger than them or than another Tommaso. A similar function is played by “-etto”.

The opposite would be in general “-one”. A “Tommasone” would be a Tommaso who is perceived as especially tall, heavy or “big” in any sense, perhaps with respect to his personality.

Those are not in general disrespectful, but obviously you wouldn't use them unless you know that particular Tommaso well and know he isn't bothered by this variant of his name.