Ojisan to Marshmallow was in no way groundbreaking, nor does it stand out among similar shows, and I doubt it will be remembered for very long. Its gimmick was marshmallows, and its main character was older and wider than main characters usually are, but that’s about it. Unfortunately, this is often the case when it comes to short anime. They have to go to absurd lengths with the jokes to satisfy viewers in only 3 or so minutes, and that usually robs them of the possibility of having any truly memorable moments. There are exceptions, of course, like Teekyuu (which commits itself fully to its absurdity, so much so that it’s basically unchallenged in its own little niche), but most short anime have to wrestle with this dilemma.
That’s not to say that Ojisan to Marshmallow was unenjoyable, or boring, or unfunny, because it was none of those things. It was a romcom with some occasionally very funny moments and a few sweet moments. I really liked it. Usually, it would have been one of my favourite shorts of the season, but as it happened it aired alongside a few other short shows that I ended up liking much more. Though Galko-chan and Tabi Machi Late Show (as well as Kono Danshi Mahou ga Oshigoto Desu and Kanojo to Kanojo no Neko) were ~7 minutes long and thus had an advantage over Ojisan to Marshmallow as far as time for jokes and storytelling goes, I also think they managed to break new ground in ways that Ojisan could not. For instance, Galko-chan was pretty gross with its jokes, which I found surprising and refreshing.
In the end, then, Ojisan to Marshmallow will probably have to enter the history books as a thoroughly mediocre show, but again, that’s not necessarily a bad thing. With a long format show, where the time investment ends up being on the order of several hours, mediocrity can make me feel like the show wasn’t worth the time, but Ojisan to Marshmallow took like 36 minutes of total watching time to finish. That’s not a lot of time. It’s like watching one and a half episodes of a long format show. So, if I had to rate my time spent watching Ojisan to Marshmallow, I’d say I had a good time, even if the show itself was mediocre. And isn’t that the most important thing?