The Clash are undeniably punk, lets get get that out of the way first. Now with the second part. The idea of "pop punk" has changed and altered over the years, from The Ramones to Rancid to Green Day to the whole new species of modern "punk" bands that include Fall Out Boy and New Found Glory ext. I think why people get heated when the term "pop punk" comes up is because they kinda contradict each other. The whole idea of punk is to NOT be pop, to create something that sits outside the mainstream. But I think "pop punk" can be a good term to use when describing a band like the Clash. Sure they were punk, but they had such an appreciation for making good songs that blended into other genres and played with a wide variety of sounds. They had no problem with experimenting, a good example of this is in their iconic album London Calling. Nonetheless, they are a very good band.
Yes, the Clash were a pop punk band, because that's how they were known. The Clash wanted to be known as a pop punk band, and most of their fans identified them as this genre. That is also how history remembers them, because the people who are fans of them today tend to be into punk.
The Clash were kind of a pop punk band. They were around when there was no pop punk genre, but if there were, it would have been pop punk. They would have been pop punk because they are a punk band and their music is rather popular. That makes them pop punk. Their sound is appealing and crosses over.
During the Clash's era and peek of popularity, yes, I would consider them a pop punk bad during that time. However, these days the definition of a punk band is a bit different as are the times. The Clash now would probably be known an "emo" band if anything else.
The Clash started the British punk band movement back in 1976. For 10 years, The Clash played to sold out audiences all over Europe and in the United States. Although not as popular as other British bands at the time, The Clash left their mark on the music industry thanks to "Rock the Casbah" in 1982. In 2003, the group was inducted into the Rock and Roll Hall of Fame in Cleveland. Perhaps their most famous song is "Should I Stay or Should I Go." Their style of rocking out on stage and destruction of instruments were ahead of their time as the punk movement gained momentum in 1977.
No, the Clash were not a pop punk band, because they were too punk to be considered pop. The Clash were more alternative. They were never mainstream. They never had a popular audience. Even today, The Clash have a very limited, but loyal, appeal. They cannot say that they were pop, because that doesn't exactly fit their genre.