I don't think it's that it lacked character development, that was actually great. It's just that they kind of threw it all away, they built it all up around this woman only to make it about Ted's love for Robin, they say that the mother was the love of his life, but they showed that it was Robin.
I think it lacked character development for several reasons:
Lily’s character development throughout the series was great but she never wanted to sacrifice her career to be a mother. We never know what happened to her career, only Marshall’s.
Barney’s character development seemed to disappear altogether. After seasons of going through major changes (finding out who his father was, realizing he truly did want to settle down) seemed to be almost unnecessary throughout the series, it was like we were back to season 1 Barney and Robin was merely another number to him, along with the mother of his child who he even referred to as “number 31”.
Robin’s character development didn’t backtrack quite as much. She was still focused on her career until the last few moments where it was as though she was never truly happy until she saw Ted standing with the Blue French Horn which is inaccurate after 9 seasons.
Ted’s character development has been faulty for the past few seasons. He convinces himself he’s in love with Robin for a day before realizing that even years after their relationship they both want different things and neither has changed into a person the other could be in a long term relationship with. It made the mother seem like a rebound from Robin which ruined a chunk of the show for me.
I think Marshall was the only one who was constant in the finale. He went where he wanted to in his career and relationship so it was nice to see that at least one character stuck with their development.
The whole show in it’s entirety was about moving on and trying the find the one and the finale was the opposite of that.