What performers today are more pop, but are advertised as rock artists?
Today Maroon five is advertised as rock, yet have more of a pop vibe to them. Their earlier work has a very light rock vibe to it but now a days it's definitely a pop vibe. Or even todays boy bands like 5 seconds of summer they look like they could be rock stars and they are known as "rock" but they are pop all the way. Personally I think it's because of who they sign with, the record company may be a company who is known for rock and that's what they had pictured, but it didn't work out.
Are any of them Britney Spears, or Madonna for example as far from being rock musicians as Pat Boone was in the late 50's and early 60's
Like everything else a certain type of music goes out of style after a while and as a trend ends a new one begins and whether the artists falls with the new trend or not is their decision. For example the Beach boys at first had a surfer vibe then changed their style more to a pop style when their albums "pet sounds" came out it had more of a pop sound to it, because the surfer style was out. As for Britney Spears she came in just as the pop trend came out, yet her name was most likely signed to a rock label. Madonna was once a "rock star" yet she just went with the sex drugs and rock and roll vibe, yet she was just truly always pop and kind of a mess.