No, Dean Ambrose hasn't been a colossal failure. People put so much pressure on these guys like, for instance, comparing Dean Ambrose to Stone Cold Steve Austin, and when these impossible expectations aren't met they act like said wrestler hasn't "lived up to the hype" or whatever. Dean Ambrose was never going to be Stone Cold. Nobody is ever going to be Stone Cold. Dean Ambrose is a solid wrestler who, despite his shitty booking most of the time, can still be inserted into a main event championship feud at the drop of a hat and have most fans not only accept it, but get behind him for it. The guy has been consistently and legitimately hot for 5 years. How many *full-time veterans can you say that about at this point? John Cena, Randy Orton, Seth Rollins, Bray Wyatt and... that's about it.
To be over at that level in this day and age and for that length of time means you're good. Really good. You have to be to keep the attention deficient fans of this era happy. I mean, the guy has been treated like a second class citizen since the Shield split and has still managed to become a 2 time IC champ, WWE champ, and get a Brock Lesnar match at Mania, all while staying extremely over with the fanbase.
People who complain about his wrestling don't get it. Here's a hot tidbit. Wrestling ability matters very little in the world of sport-entertainment. It's why Sami Zayn, despite being a great wrestler, is not very over with the crowd. Guys like AJ Styles and Kevin Owens might be fantastic wrestlers, but that's not why they're over despite what some may have you believe. Dean Ambrose objectively is one of the best commodities in the WWE right now. Is he John Cena? No. But nobody in this era is comparable to John Cena.