By almost any other band's standards, that record would have been a success.
By U2's standards, world wide sales of 8 million were a relative failure, when you compare it to 25 mil for JT, 18 mil for AB, and 13 mil for R&H. Even Zooropa, which was basically an after thought, sold 7 mil with almost no promotion and a hugely successful tour going on at the time. Of course, 8 mil would be considered pretty exceptional for them right now, but Pop was still the pre-MP3, pre-pirating era when U2 was at the height of their popularity.
Add to that they were having trouble filling seats for the tour in a lot of North American markets even with deeply discounted ticketing (the radio station where I lived in San Diego at the time, and saw the show, was giving them away to anyone who showed up), it's hard to call this record a success by U2's standards. Whether it was an artistic success is of course another matter entirely.
And since the bands statements about the record over the years speak for themselves, I'll stop speaking about it.