I'm not so sure
Even back when she was little more than a brat she went against what the other Diamonds wanted, despite them repeatedly punishing her for that behavior. When she went down to Earth and decided on her own that what they were doing to the planet wasn't right when words failed to convince the other Diamonds she started a war over it, and while a coward might start a war, they sure as hell wouldn't be willing to fight in it. But she was, despite the fact that if she ever got poofed everyone would have seen that wasn't a quartz Gem and her ruse would be up. She also did all this while holding back her destruction wave power, as she likely didn't want a repeat of what happened to her first Pearl. These aren't the actions of a coward.
However, she did all that while hiding behind a persona, she didn't fight for what she believed in as Pink Diamond she did so as Rose Quartz, and that is something a coward would do. We see this duality again when bismuth insisted on using the breaking point a coward would have just gone with it, but she stuck to what she believed in and fought bismuth. Yet like a coward, she hid what happened likely fearing the effect it would have on her troop's loyalty and morale.
So I'll argue she isn't cowardly but she isn't all that brave either. I suppose the closest word I know of to describe her is steadfast if she thinks that something needs to be done she will do it even if it might end poorly for her, but she doesn't go all in. A lot of people say she had Steven as a way to pawn off her responsibilities, but I don't see it that way. I think she chose to have Steven because she wanted to change to be a better person but found herself too incapable of doing so. So she concluded that the world would be better off if she was replaced with a being that was not only capable of the change she wanted but was able to do so quickly in her eyes.>>111982732
No, I'm talking about the 3 Roses that visited Steven