Are "non-fat" condiments as effective as full-fat versions at reducing spiciness?
It's the fat that actually cuts the spiciness - see my answer on making a sauce less spicy/hot. When you're dealing with the type of spiciness that's reduced by cream (generally capsaicin), it's pretty safe to assume that it's fat-soluble, and the same principles apply when you're using the "reducer" as a condiment vs. including it in the recipe.

There's certainly a possibility that certain non-fat/low-fat substitutes will work, but it's going to be luck of the draw; it depends on how they're made. Alcohol in general is almost as good as fat at cutting spice (sometimes better), but that's not going to be a common ingredient in these products. Generally they'll have a bunch of emulsifiers and other E-numbers to maintain their consistency, not many of which (possibly none) have any documented relationship with capsaicin or its like.

Low fat (as opposed to non-fat) might be fine because it still contains some fat. "Light" sour cream is typically still 5-10% fat and would probably still work pretty well, if not quite as well as full-fat. But don't expect any miracles from non-fat products.