The answer? Yes. Mamoru and Usagi to be together, especially in the manga, need to be more than just lovers - they need to be companions. In the manga, all they need is each other to be happy. The anime didn't show us their love and happiness as much as it happened in the manga, but it still shows their closeness. Take for example the Super S Movie of Sailor Moon. Usagi is talking with Luna and she says that when she and Mamochan are together they talk, and that Mamochan gets so excited when he is talking about what he is studying that afterwards, he falls asleep on her because, as Usagi explains, he is so comfortable with her. When they are talking and behaving this way, they are not lovers, they are FRIENDS - they are in the act of TALKING, not hugging or kissing, they are just together as friends would be spending time together. Plus, the very fact that in the anime, he falls asleep on her is proof of great trust and security - something that is found in best friendship and in love.
Our couple has had a tough time becoming friends though. You can see how their relationship develops in the DOOM TREE series of Sailor Moon. How through helping the baby they babysat for, how they battled together in the arcade, and especially how Mamoru reached out to Usagi in the last episode of the Doom Tree series, trying to save her, how CLOSE they had gotten as JUST Usagi and Mamoru.
Then we can see in the opening of the R series that Usagi and Mamoru have been spending time together a lot, falling love with each other just as they are. He calls her "my klutz".
Furthermore, for a couple like Usagi and Mamoru to be so in love with each other and to be together for as long as they have been, they HAVE to be best friends as WELL as lovers. Ask anyone who has been in long-term relationships and they will tell you that the "lover" aspect is not what is most important and can fade anytime...what is MOST important is the bond of friendship and the love that proceeds from that. Without the friendship, where is the love? How can Usako and Mamochan love eachother yet not be close friends? They tell each other everything, confide in each other about their worries, are there for each other in the worse times like when Mamochan was sick with the black stuff in his lungs - he was being poisoned by the curse of Elysion and Usagi was right by his side - that is not only the mark of true love but the mark of true friendship.
With friendship comes happiness, love, and appreciation for each other. Lots of people say they have fallen in love with someone, but after spending time with the person the relationship ultimately come down to the test of friendship. Is the person you're with someone you can talk to, someone who is a friend and not just a lover? Lots of people find out, after spending time with the person they are "in love" with that they are no longer in love. By getting to know each other they found they didn't really see eye to eye, didnt have common goals or views, and essentially, weren't friends. Without friendship, how can their be love?
In contrast, Usako and Mamochan are tested in FRIENDSHIP many times. In the Manga volume 15 they are trapped in dreams and both wake up after realizing that their dreams are one and the same - to help save their planet. With this realization, they take each other's hands and you should SEE the bond of both friendship and love there.