The new Star Wars trilogy was primarily about Rey and Kylo Ren, they were the lead characters, but, there is really no excuse for the treatment of all the other characters, Finn most of all.
They put great actors in every role and then did either nothing, or gave them crap plots.

They had a female lead, which was great, and hired a diverse cast, which is great, but what's not great is how they were ultimately used.
It could have, should have, been better.

There's a lot of blame to go around for that.

It was fine for the focus to be on Rey and Ren, but TLJ sidelined Finn, quite literally, forgetting that these characters worked best together and keeping them apart was a huge mistake.
TROS put them back together but didn't know what to do with them.

Not planning the trilogy was the biggest error.

And maybe there was a plan at one point, we will never know. We got what we got, and there is a lot of good in there, I'm glad we have them, they should have done better.