Well?
U2 are still a rock band like they were when they first started. This term "Dad Rock" is meaningless because it has multiple definitions many of which contradict each other. If you don't fit into the narrow box of what some perceive as indie'/alternative rock, then you could be tagged with the label.
When you think about it, any definition of anything is subjective, relative and culturally situated. Additionally, definitions vary over time. A band like Radiohead, for example, could neatly fall under the definition of ‘rock’ up to, and including, The Bends. OK Computer could usefully be considered ‘art rock’. On the other hand, an album like Kid A falls more neatly into the category of ‘electronica’.
U2, since POP could, depending on the song, accurately be defined as ‘rock’, ‘pop rock’, ‘soft rock’, ‘dad rock’ or simply, ‘shite’.