Yes, a lot of the Marceline stuff - her and Aberdeer, her and Bubblegum, her and Simon - came in part from Sugar (episodes co-written by her and another writer), and the songs definitely did, and Steven Universe absolutely has that, like, very real-feeling strained interpersonal emotional core under a layer of fantasy adventure thing feel to it.
If you watch Steven universe, and then go back to watch early seasons of Adventure Time, you can definitely feel her influence on Ward's show, and, sadly, can feel the lack of it in the episodes after she left. But considering that she left to work on this, if anything, even more amazing show (personal tastes may vary), I feel it was worth the trade, and I still feel Adventure Time will find its footing, even if the last season was, imo, a bit shaky.
And heck, given how much leeway Steven Universe is
somehow managing to find in the
queer representation thing (seriously, considering how non-negative representations of queer characters, especially in children's shows can get your show banned in several overseas markets, its frankly remarkable what Cartoon Network's been ~spoiler warning, though honestly the vid linked in my previous post was just full of spoilers ~
letting Sugar get
away with), if it continues to be successful maybe the Adventure Time crew will finally get the ok to push some of the backstory and character stuff they've been implying but never been free to tackle openly with Bubblegum and Marceline for years now.