I am hopeful Ugo has improved its just hard to expect it since most players dont under Cal.
Let's look at your statement factually (over the past 6 years).
First have to narrow down to multi-year players to evaluate if they did improve under Cal or not.
Oscar - no, but not really reasonable to expect significant improvement over NPOY
Toppin - yes some, but not significant improvement
Ware - no
Wheeler - no, but many thing he was already at his ceiling
Collins - no, but we don't know what would have been different had his dad not died at season start (skinny kid loses 20 lbs, clearly affected him)
Mintz - no, but was probably already at his ceiling coming in for 4th & 5th years
Brooks - yes some, but not significant improvement
Allen - no, he got worse
Hagans - no
Quickley - yes, significantly better
Richards - yes, significantly better but it took 3 seasons instead of 2
Montgomery - no
PJ W - yes
Gabriel - no
SKJ - no
Mixed results. Supporters would point to Quickly, Richards, and PJ. But there are more players that support your theory.