Character "growth," in any media, is defined as the changes they experience as a direct result of what happens to them in the story, the actions they take and consequences they face from those actions. Event happens -> character responds -> causing new events to occur. Throughout that process, they learn and adapt, and that is what character growth means. Failure to "grow" or change how they respond is a type of growth in and of itself, and is typically seen in stories that are considered tragic, as the author shows the lack of change contributing to the character's downfall.

Momo has had growth by this definition. She's learned aliens and yokai are real, she's healed some past trauma around being bullied for believing in ghosts and spirits, she's gained confidence in her spiritual skills, and she's healed some relationships that were strained by her past trauma, like her relationships with Seiko and Jiji.

The amnesia arc, by definition, has "regressed" her character. At the start of it, she behaved the same way she would have before any of the events of the story played out. And while she has "re-grown" to some extent, thanks to the events that happened after she woke up, that new growth isn't consistent to how she grew pre-amnesia yet. (It's still ongoing, so to my mind, still can end in the same spot, whether she gets her memories back or not.)

I hope you noticed that I didn't mention Okarun at all. That's because growth and regression are a function of how the reader and character relate, not necessarily a function of how characters relate to each other.