This is a minor point, I realize, but "catsup" is still a fairly common, if old-fashioned, alternate spelling in English of "ketchup" and probably should be added as an allowable translation for "ketsuppi" in all relevant questions.
Because it's a general statement about ketchup, and not, for example, "some ketchup", the basic (nominative) form, "ketsuppi", seems correct here. I'm sure a native Finnish speaker can better elaborate, though.
You will get used to it. Finnish has peculiar situations like this where it's the nominative case, even when that may seem counter-intuitive. General statements about weather, for example: "On kylmä" (as opposed to "kylmää") for "it is cold."
Spicy in the sense of containing clearly tastable(?) spices is mausteinen. Spicy in the sense of being hot, too strong is either the same mausteinen or voimakas/voimakkaasti maustettu, strong/strongly spiced.