The totality of available scientific evidence supports that intake of milk and dairy products contribute to meet nutrient recommendations, and may protect against the most prevalent chronic diseases, whereas very few adverse effects have been reported.
Edit: it's been a long day.
(I guess the answer depends mostly on what you think about saturated fat and sugar. If you believe both of those are fine, ice-cream is A-OK. I eat it a lot and seem to do alright.)
(At least, ice-cream in the traditional sense -- milk, sugar, maybe eggs, whatever flavors. Sadly many of the ice-creams you'll see on the shelves these days have all kinds of weird fillers and stuff; whether those are healthy or not is a whole 'nother question.)
A similar data point is men of the Bodi tribe, normally quite skinny, have a contest to see who can get the fattest during a 6 month period: the technique the men use to get fat is to eat a lot of blood and milk.
So dairy definitely has a lot of weight-gain potential, if you eat enough of it!
(Though interestingly even the fattest Bodi men are still not as fat as the people you'd see any given day in, say, an American Walmart.)