Hello, Why does Rodimus' appearance in Headmasters look a little different? I think I have seen an explanation regarding the matrix but I am not sure. And why was Rodimus so popular in Japan?
Real world answer: slightly different animation style. While Headmasters largely used the Sunbow models and styling, it still reflected the fact that it was truly anime (in the sense of intended for a Japanese viewership) rather than American-style animation for US kids. It's basically an anime copy of the Sunbow/Marvel Productions style. I'm working my way though all the TF shows in order and just finished HM. My guess at an in-universe answer: The Matrix altered his Rodimus Convoy form to better reflect maturity he'd gained in his first round as Commander.
Well, I dunno if popular is a good way of putting it, since in Headmasters, Roddy only got a brief tenure as a leader before Cybertron was destroyed and he just left. In fact, I felt that the Japanese wanted to shove Headmasters so deep down our throats that the only people allowed to save the day were the four jerks from Planet Master (and if it wasn't them, it was Daniel).
Season 3 of the cartoon was largely animated by South Korean studio Akom and the quality was... spotty at best. Headmasters returned to being animated largely by Toei, who animated the movie and most of Season one and two. So part of it is simply animation quality. However, in addition to that, instead of the Floro Dery character models that were used for the US cartoon, the character models for Headmasters were designed by Ban Magami, the artist for the manga. This includes redesigns for characters that had appeared in the earlier series, so there would be some differences. This is just a theory, but considering that 1.) Japan did not originally get the movie and 2.) what we got as season 3 in the west was actually treated as a brand new series Transformers 2010 (think like Dragon Ball and DBZ) in Japan, means that Japan avoided the trauma that 80s kids in the US experienced with the film, and just a general favorable outlook toward the main hero.