I think it's pretty unfair how fans are expecting her to get over not having friends growing up. Not having friends growing up can deteriorate people's social skills and self-esteem, which is why Starlight is the mean evil villain she is.
Now I can agree with the fans that Starlight's backstory needs a lot of improvement because it did seem kind of weak and rushed. It's not realistic for anybody to become evil and angry over one single lost friend. It takes more than just that to become a villain.
For me, there are some friends that I lost and I didn't cry or get upset. Heck, I even moved on quickly from some people. However, I didn't have many friends growing up and my classmates bullied me for being unattractive and weird. I'm a female and I had a difficult time making and keeping female friends. I still don't have many friends as an adult and I've been backstabbed, bullied, and humiliated a lot. Now I've never wanted to go crazy and start a cult like Starlight did, but I'm unhappy and angry as a result and sometimes tempered to just not be a nice person because of it.
Here's a question for fans: if Starlight kept on trying and trying to make friends and she was constantly rejected and she was constantly bullied, would her backstory make more sense to you guys?