Earlier quoted context omitted.
If 'mostly' means over 50% you could call this clear. Otherwise I don't think this is clear at all. Otherwise, what is mostly? Mostly at the outset, or at any given time? Must the project move out when it goes from 49% to 51% AI-assisted code?
"Mostly at any given time?" Yes, old project with 1M LOC of pre-AI code + 100% AI 100k LOC generated coded for the last three months, is that mostly?
"Most" is clear and unambiguous.
It means more X than Y. The value of X is larger than the value of Y.
X = bot slop ; Y = human written code
In your question, 90% human written, 10% bot generated; no, it is not "most".