Not trying to be snarky or argue (since sometimes things get taken the wrong way), but what makes it underrated? I'd love to be sold on why this movie is better than I thought it was (I admit I could have had the wrong expectations towards it, maybe it's worth a re-watch; I remain unsurprised it tanked - Disney standard tanking, not regular tanking).
I'll bite. Firstly, we all have different tastes and there's nothing wrong with that. The film just might not have been one that connected with you. It certainly didn't connect to a wide audience.
I can only speak to my own opinions on the film. To me, there was far more to like about it than dislike and, in some cases, there were aspects of it that I found to be extraordinary.
The production design of the Tomorrowland city was top-notch. You could tell that a lot of thought went into it.
There was a good balance of action and ideas. Too many times, "family films" aim low and are simplified in order to reach a wide audience. Like the best Science Fiction, Tomorrowland dealt heavily with an idea and showed a possible outcome of moving that central idea forward. In this case, it was the Ayn Rand-ian notion of Objectivism. What would happen if that philosophy was taken to an extreme? What are the benefits? What are the undesirable results? In the end, is it worth it? That's pretty heady stuff for a family film and tricky to balance with lighter, more traditionally entertaining elements.
I thought the acting was good. I always like George Clooney and Hugh Laurie, but I thought the young actors did a fine job as well.
It had interesting action beats, though I do think that the last section of the film seemed to sacrifice some of the previous thoughtfulness in service of ramping up the excitement.
It was shot well. Camera usage was creative and aesthetically pleasing.
I guess, overall, I just thought it was a notch above most summer movie fodder. While it had some story faults and felt like perhaps it had been "tinkered" with by a studio that didn't trust the material, the bulk of the film was thoughtful, heartfelt, and exciting. I think it could have spent more time in the titular Tomorrowland, but I don't know how much of that is the fault of the original script or post-production editing.
In short, I found it to be a fine film with a lot of creativity and that, even with its faults, it deserves far more credit for being a flawed, but still really good film than it has received.