Icon is a good number of enthusiasts favorite coaster. The ride isn't a flop, its pretty widely enjoyed. It is the marketing & promotion of the ride which was (and still is) a flop.
I personally like Icon a fair amount more than most... and even I'd have to disagree with that statement. As much as I personally love it, I accept that that is quite a minority opinion these days.
If we're talking in terms of general reverence, I'd argue that Icon was
definitely a flop, or at very least a disappointment compared to common expectations. In the months before it was built, there was lots of hyperbolic talk about how it would be "the best coaster in the UK", how it would "have Europe and the world looking at Blackpool in jealousy", how it would "annihilate Alton Towers' poxy little family GCI", how it would be "the UK's answer to Maverick at Cedar Point", how it would "represent a new dawn for Blackpool Pleasure Beach and blow the UK coaster industry wide open" and numerous other hyped comments. The hype for Icon was off the charts compared to any UK coaster before or since, and before it opened, there seemed to be a common consensus that the ride would blow the UK coaster industry wide open and give us a modern, universally revered ride to compete with the various European heavyweights.
When it opened, I think it's fair to say that none of those things happened. There were some very positive reviews initially, but these faded away over time. While popular belief at the time was that Icon would endure and be revered forever more and Wicker Man would be a flash in the pan that was only liked for the first few months, I'd actually argue that the opposite occurred. Love for Icon seemed to absolutely fall off a cliff after its first year or so. Don't get me wrong, the ride has its fans, but many seem apathetic towards it at best and actually quite negative towards it at worst. I dare say that even for many of the ride's biggest fans (a group I count myself among), it is more of a "UK-class" ride than the "European-class" or "world-class" ride that everyone expected it to be before it opened. Even among people who relatively like Icon, you don't often see it being a list topper once the scope of said lists expands beyond our shores. I love Icon to bits, I think it's a phenomenal ride, and it is my UK #1, but even for me, it's on the precipice of being turfed out of my top 10 this year (in the #9 spot, to be exact). And I should add that I've only done 111 coasters, which isn't exactly a massive total compared to some.
I can't speak too much for non-enthusiast reception, as I live nowhere near Blackpool and the park isn't too well known or visited down here in Gloucestershire, but in terms of my own family, it was a mixed picture. My sister absolutely loved it and declared it her new #1 when we rode in 2018, but my parents rode it with me in both 2018 and 2019 and were both largely apathetic towards it. My dad, a man who rode the similarly double-launched VelociCoaster in Florida last year, instantly declared it his new #1 and called it "an absolute revelation", said that "The thing that impressed me most about Icon was the cool baggage system in the station, which speaks volumes about my thoughts on the actual ride". My mum, who doesn't even overly like ridiculously intense rides (she doesn't like Nemesis or Oblivion, for instance), said that "A good coaster makes me laugh and scream lots. Nothing about Icon made me laugh or scream. I didn't particularly like it". I know that for both of them, it's not even Blackpool Pleasure Beach's strongest coaster. I know for a fact that both of them far prefer Wicker Man. And to be honest, I myself am not that far off preferring Wicker Man... it only sits a spot below Icon for me, and on a particularly good day for Wicker, I could see an argument for it narrowly usurping Icon.
In terms of its
commercial success, I'm not sure if Blackpool would necessarily have had automatic success with Icon had they built something more widely revered. They could have built the best ride on Earth, and it may have made little difference if they'd marketed it the same as they marketed Icon. With that said, it may have helped, particularly if the ride had been a bit bigger and more impactful to sell itself a bit more.