Okay, Civil War wasn't just about Bucky. It was about Wanda too.
Sure, Cap was after Bucky, and half of the movie was wrapped around those two, but don't forget Wanda.
Wanda lost her brother may be a year (give or take a few weeks or days) before and is still probably reeling. Remember the beginning shot where they're in Lagos? Well, she then went from that to killing who knows how many (at least 11 Wakandans). So, it's been a rough year.
And because of all this, she falls into self-doubt, self-hate, and probably depression. Not even Vision can really help.
Soon after, Theadus Ross comes up with the Sokovia Accords, partially influenced by her actions in Lagos. And then, Tony Stark locks her in the compound. And his excuse?
"She's not a US citizen, and they don't give visa's to weapons of mass destruction!"
-Tony Stark to Steve Rogers. Captian America: Civil War
Really Stark? And then, he goes so far as to leave her, get Peter, and have Vision be prepared to kill her if she tries to escape!
When we see her after Hawkeye rescues her she's at the airport in Germany. And what does Iron Man do? He cracks a joke.
Sure, locking up a teen girl is jokable, only to Tony Stark.
A little later, when she's locked up, she's seen in some sort of collar (which I assume is a shock collar of some sort). Plus, she never talks to him, so it's most likely that if she talked, she got zapped.
She's been through so much. I mean, just think. She's a right in my age range (13-19) and she gets locked up in a max security underwater prison.
And yet, even with everything else in the plot, we still get the feeling, this is about her. The Sokovia Accords probably wouldn't have come into effect without her mistake in Lagos. She is a really big deal in this movie. Her role may not be as big as Bucky and Steve and Tony, but hers is just as impactful and important.