Honestly, this was the reverse of several other manga I have read recently: started a kind of rough / weak, but then got its footing and ended up closing really strong. Where other manga have lost points from me in their concluding chapters, the last several volumes of Ashita no Joe more than make up for the beginning.
My biggest issue with the beginning (first 15-20%) was with characters' motivations, especially Joe's and Danpei's. At times it felt like they were doing things just to advance the plot or create funny moments, rather than because of any clear personality or principle. While looking back you can perhaps read some of these moments in light of what Joe's principles were later revealed to be, it still seems like a good bit was the author needing some way to advance the plot. Joe was also kind of frustrating because of his whole "rebel without a cause" schtick. Some of that made more sense later and became endearing, but some, to me, remained unnecessary and made him rather unlikable for me for probably the first 1/3 - 1/2 of the manga.
The early art quality also was not the best, and I am not sure the author (perhaps understandably) knew exactly where he was going with characters or themes. Pacing felt off, and a lot of dialogue seemed repetitive and/or like filler. Also, not the mangaka's fault, but the early translations here are pretty rough.
However, as said, things started getting good somewhere around 1/4 - 1/3 of the way through, and really good somewhere between 1/2 and 2/3 of the way, and closed phenomenally. The comedy took its proper place in the background and tended towards the more realistic. The boxing and training became more meaningful (early chapters overplayed the power of simple moves and cross-counters were over-relied upon). All (or almost all) dialogue and action felt meaningful. Pacing was exactly on point or perhaps occasionally a touch fast (but much better than the slog of early chapters). And best of all character development and the development of overarching themes and philosophies skyrocketed.
Even at 1/2 way through I was not sure I was going to finish this. So very glad I did. I happily give this 10/10.