Dull is 100% subjective. Expecting everyone to agree on what fighters are dull and which are exciting is like expecting everyone to agree on what's the best music (rock, classical, jazz, hip-hop, country, folk etc), or the best movie (drama, action, documentary, animation) or even the best color (red, blue, green). No one past grade six really believes that people are going to agree on subjective things, so I'm assuming anyone who thinks people agree on whether a particular fighter is either dull or exciting is trolling (or in grade six). Some people obviously found GSP dull. Other people just as obviously found him entertaining. That's how human nature works.
However, I completely agree that GSP is old, and he's had one fight in six years. I can't imagine why even his fans want to see him fight at this point. It'd be like Shaq O'Neil's fans (and Shaq has way more fans and was a way better athlete than GSP ever was) wanting to see him come back to the NBA.