I think we all agree that Mamoru was most well handled in the DK arc both manga and anime. In the manga he and the romance was the main focus. In the anime he was shown well as a cool independent character with interaction towards everyone rather than just Usagi's lover. The romance wasn't that focused but the big climax of Usagi transforming in the elevator and Mamoru awake/dies was very touching.
But since R the problem came. Both Naoko and Toei originally wanted to end the story with DK arc. With everything revealed, there is little room for their relationship. So we saw Chibiusa to create some funny chemistry. The manga let Inner Four got captured so early, but still Demande was the most well characterized male in this arc, I think Naoko had to let him died with a rush to keep Mamoru's status. The anime didn't let Mamoru recover his memory until the end of the filler arc, An having a crush on him, even their "part away" was to create more tension and drama for Usagi/Mamoru. Also to focus on Mamoru, Demande didn't play a big role until the end of the anime and his crush on Usagi wasn't explained that well compare to the manga. The R movie created Fiore for the same reason. Some of these tries were good some were bad.
Since S, Mamoru is no longer the focus of the anime, even SS didn't pay much attention on him. The manga treated him a bit better but not much. The story just couldn't create more because all the drama has been overdone.
Overall he suffered from being Usagi's lover so after their relationship was built, he got little room to start independent arc, Michiru also suffered a bit from similar problem. Mamoru also played "Distressed Dude" role way too much to create tension for Usagi so this had left ppl negative impression.