I headcanon that younger Tia was good-natured, but didn't fully understand the consequences of her actions. She liked pranks and jokes, but as she got older and more powerful they would become more extreme. When somepony would told her she was going too far, she would scoff and tell them to lighten up. I like to headcanon that as Luna grew more bitter about being forced into the background, Celestia didn't take Luna's complaints seriously because Luna would lash out in similar ways when her older sister would tease (read:bully) or prank (read:punk) her. Since Celestia's mentor was Starswirl, who is himself emotionally absent, and due to how powerful and important she is and was becoming, Celestia would lack empathy in her teenage and early adult years until Luna let herself become Nightmare Moon.

I choose to headcanon that Celestia would have become a giga-Stacy and a ruler similar to Sombra if Luna hadn't gone to extremes, and I also choose to headcanon that Celestia banishing Luna was her first real wake-up call at realizing just how far-reaching the consequences of her actions are. I know that many others want Tia to have always been a kind, gentle-hearted pony, but for me, she's more appealing if she was a hair's breadth away from becoming a villain.

It adds tremendously to both her character and Luna's that way, in my opinion, because it means that Luna wasn't being petty when she decided to become Nightmare Moon. She was doing what she thought was best for Equestria by attempting to overthrow Celestia because Luna saw what her sister was becoming, but she tragically didn't realize the full consequences of becoming Nightmare Moon. Celestia similarly wouldn't have had an impetus to change because everypony was singing her praises every day, and no one was challenging her because no one realistically could. Having Luna stand up to her would have been the wake-up call Tia needed to finally grow as a pony, but it sadly came at the cost of her sister.