It's my personal and objective belief that what is cute about the Steven Universe's gem Spinel is that:
She has a simple but adorable design, complex in it's little.
Relaxing colours and comfy clothes that give to the watcher a sense of comfortableness.
A lovely and big heart of gold that makes you want to support her, cheer her up and love her.
Her backstory is relatable to a lot of people that experienced abusive and uncaring relationships with members of their family or people they believed were their friends. Many people see themselves abandoned by Pink Diamond in their Garden and so they want to cheer up and hug Spinel because nobody was there for them, so they want to be there for Spinel in the same way they wished someone was there for them.