The central theme of Majo no Takkyūbin is about self-belief and being yourself. This message is explained to Kiki early in the film when she complains about having to wear the drab witches' dress. Her mother explains that it is her inner self which is important; her outer self is just a shell.
Even before she leaves home, people question how Kiki will be able to survive as the only thing she seems to be good at is flying (and even that is a little suspect). And after she leaves home, she is repeatedly plagued by more incidents which batter down on her self-worth. These include:
- Meeting the snooty apprentice witch who pooh-poohs her skills.
- Meeting girls wearing pretty colourful clothes who look down on her.
- Even Tomba comments on her clothes at one point.
- IIRC, she also stares longingly at a dress in a shop window.
- She inwardly does not think much about her chosen profession or her skill level either.
- She feels unappreciated and underwhelmed by the response of the granddaughter to the freshly baked cake she has flown over in a storm.
- She also feels really out of place when she meets Tomba's friends.
etc., etc. (Personally, I think Jiji's negativity either doesn't help much or is a reflection of Kiki's inner self.)
All these incidents chip away at her inner self until she collapses into a depression, something which her artist friend, Ursula compares to an artist's block. She explains to Kiki that she has been through a similar situation herself when she lost interest in her art as she was modelling her approach on what other artists were doing or what her audience was expecting rather than believing in herself to come up with her own style.
This brings us back to the early scene where Kiki's mother tells her that it's her inner soul which is important.
All that said, this is also a coming of age story, and as the OP puts it, the loss of power could well be something that all witches go through as they mature. The fact that Jiji doesn't speak to Kiki even after she recovers her powers is telling. In many ways, it is only the childish Kiki who is able to speak to her childhood companion. Kiki, the mature independent witch, is still friends with Jiji, but does not need the cat as a confidant.