Ok, let's take a look at each of top opponents Ajax played against to date.
1. Bayern Munich. Except for them and Ajax, there were Benfica and AEK in the group - not the hardest job for both teams that went through to the round of 16. By the end of the group stage, in Bundesliga they were on the 3rd place (it's as if Juventus were behind Napoli and Roma), with the goal difference worse than RB Leipzig's and Eintracht Frankfurt's below them. In 1/8 Bayern was humiliated and eliminated by Liverpool.
2. Real Madrid. On the group stage they had Roma, Victoria Plzen and CSKA. Of course they got to 1/8. By the end of the group stage, in La Liga they sat on the 4-5th place, having lost third of all games. At the time of the round of 16 they significantly improved their results in domestic league (2nd place), but were still shaky, which eventually led to the coach replacement. Nonetheless, they pretty confidently won the away game against Ajax and clearly underestimated their rivals (were unfocused and amorphic) in the return leg (and, as an addition to all these "bonuses", Ajax were very lucky on the pitch in Madrid).
3. Juventus. Since we're on Tuz, I believe, we can skip this part as obvious for everybody.
I don't see a point in saying that all these teams were in at least normal shape and their unusually bad state didn't contribute heavily to the Ajax's success.
We're still yet to see what Ajax are capable of when facing a top team in the best shape.