Right-on! I first saw this back when it initially came out on home video in the early 80's and loved every minute of it, regardless of what people may think of it from today's standpoint (being a "packaged feature" and and other stuff). I often wonder if MST3K took some of it's inspiration from Edgar Bergen's clever repartee with Charlie McCarthy and Mortimer Snerd during the Mickey & The Beanstalk part. I thought of that the first time I watched that show, thinking I saw that sort of thing done elsewhere.
Getting back to HTTYD3, I suppose had they kept the same look of a Night Fury for the other dragon, such as in the poster, it might've made the film a bit confusing for people to follow, since they may not be able to tell them apart (which one's Toothless in this case) unless they had to give some visual cue to set the mate different from the other (as long as it's not a primary sexual characteristic). Such visual cues such as differences in color, shape or other features was what set animation/cartoons apart from live-action (for me, that moment in the live-action 101 Dalmatians film where they had to check underneath the dog's hind legs is one example of the limitations of reality).