In my experience, it’s less that they’re boring main characters and more they’re boring because they’re main characters.
My main, Roxanne, she’s not terribly interesting or exciting. Cassandra is the aggressive giant dork, Annika is the temperamental caring one, Priscilla is an arrogant know-it-all, and Hazel is the ridiculously adorable kid who’s a little too desensitized to violence.
But Roxanne? She... she works in an arcade and got saddled with a leadership position amongst her group of witches, none of whom respect her. She’s a smart ass, but often keeps silent and likes to appear unfazed by things. Overall she’s bored and fairly boring, which is what I always pictured for her but it’s also hard to work with.
At one point during my planning process I considered making her the Decoy Protagonist to focus on another character later. But I decided against it.
I realized that since we more of less see things from Roxanne’s perspective, or focus on her, we just get used to her faster. Meanwhile the other cast seems a lot more interesting in comparison.
So then I figured, well, give Roxanne a core principle that borders on fixation. In her case, tho not immediately obvious, is Individualism. She firmly believes in it to the extent she believes capability trumps everything else - like age, so she doesn’t treat Hazel or Priscilla like children even tho she’s an adult. She sees weakness as a personal problem that must be overcome personally, she’s fine with helping others and getting help from them, but firmly believes people are at their best when they’re strong on their own - not relying upon others.
This sets up something important for her in the future as well. Roxanne’s self of self is what makes her less boring. She doesn’t feel the need to group herself with others and define herself by said group. She’s a leader with terrible charisma, but because she holds the individual in such high regard she’s actually more compassionate than you might think.
So anyway, that’s how I ended up going with Roxanne. The way she was before? Boring. Now? She seems boring on the surface but there’s a lot more going on.
Also I thought to myself “what’s one quality I wish I could see in more characters / people irl?” It was Individualism, but one that was more inspiring and not arrogant.
Characters are hard. But odds are your character isn’t as boring as you think.