I don't watch their league games honestly speaking. Just looking at the numbers and they are ridiculous. But every year I watch them in the CL, and yeah, again, they play excellent football, punch above their weight and are more unlucky than anything.
Europa League performance after dropping out, yeah that's a fair criticism. Can't say for sure if they just didn't pay much attention to it or treated it like United did in 11/12.
Adapting tactics successful did mean they win. They won their league every year. They won their cup in 2/3 years and made the semi final the other year. This year they are 4 points in the lead in the league and in the final of the cup. They have had domestic dominance since he's taken over. You may be tempted to say "it's Ajax and the Dutch league", but they hadn't won the league in the previous 4 seasons, and hadn't won the cup since 09/10 (8 seasons without a win). They lose all their best players every year and they still put up stupid goal scoring numbers in all competitions every year.
With their CL performances, as I've said. They hadn't qualified past the group stage since 2005/06, then 02/03, then 96/97. He wasn't in charge of a big, rich club loaded with superstars who was expected to qualify.
With regards to "who is to blame", well, it always varies and the reality is always shared. Maybe Ten Hag could/should be more pragmatic in some clutch moments in the CL. The record the past 4 years would suggest so. But it's impossible to ignore that in the first place, he is dealing with a much lower quality group of players than the majority he is up against. Its also impossible to ignore the actual performances, or just a simple stat check goes a long way too. Looking at how they still go toe to toe against European giants, how they still have good results against European giants, but maybe just don't have enough quality to ultimately get over the line.
When it comes to Pochettino... I don't see a manager there who was able to adapt regardless of the group he had. He had a good group, kept them together for a few years, but was unable to improve them enough to win anything. Pochettino like ten hag did feck all in the Europa League. Pochettino's 5 years in charge had them exit at the r32 stage of the Europa League, r16 stage of Europa League, knocked out of CL group and then r16 Europa League, exit at the r16 stage of the Champions League and then finish as CL runners up before he was sacked in 19/20. So there European performances aren't actually different at all, except well one was in charge of Tottenham, the other Ajax, world's apart financially.
Outside of Europe, Pochettino never changed any tactics. He came in with 1 idea. It coincided with a very strong team clicking together. They had a few decent league seasons and did nothing in Europe, but ultimately didn't win anything domestically either (or get particularly close). And then because he failed to ever adapt or implement any changes, they dropped down a level and he couldn't lift it back up again. He goes to PSG with the same idea. Doesn't really do anything of note. Doesn't matter the player quality. Doesn't matter what signings were made. It was the same idea/tactic and they couldn't adapt to a different group of players, or find a way to actually step up to be truly competitive.