He does mention the seal to Goku though, and then both just forget afterwards.
Yeah, he picked the "seal" and somehow didn't look at what he had gotten until he was using it, which is pretty dumb too.
In the manga, the motion sickness is specifically noted as happening when Goku looked outside in time machine. It only happened in the first trip and then the last one since was holding onto from outside.
Manga Goku also outright told Vegeta to kill Black once Trunks pointed out that he was planning something while Vegeta beat him up, rather than playing around with him until he powered up like anime Goku did, and he tried to stop Black's and Zamasu's fusion rather than just watching it. He played around with Future Zamasu since in the manga he wasn't even at Trunks' level.
They took turns against Merged Zamasu in the sense that each one attacked at once by themselves and then afterwards stood up to try again separately. The only "formal" turn that they take afterwards is Goku telling Vegeta to step aside and let him handle it, which makes sense considering how he still has a senzu at that point and there was the Mastered SSB reveal in the next chapter.
He mentioned the seal at the end of Episode 62 and went to make it. In the next episode, Goku is holding the jar talking to Roshi and thanking him. The entire time, Roshi never gave him the seal. And like I pointed out, Roshi could have stuck the seal o the jar or written the seal directly on the pot like Kami did back in the 23rd tournament. Which is why I said Roshi is partly to blame in both versions, although a bit more in the anime since at least Goku grabbed something in the manga.
True, but in the anime Goku didn't get motion sickness even on the last trip when he was hanging on for dear life. Again, motion sickness has nothing to do with intelligence and I personally get peeved when people hold this against Goku, but people used this to say Goku is dumb.
Goku really shouldn't have to tell Vegeta to kill Black since Vegeta is more ruthless than him and he got upset when Vegeta revealed that Goku wasn't going to get a turn regardless. I will give Goku points for trying to stop the fusion, but that is kind of undermined by the fact that Vegeta became the dumb one in that scene since he apparently forgot what fusion was and didn't know what was going on. So Goku became smarter there, while Vegeta became the dumb one.
As for Future Zamasu not being strong, that really isn't the point. The point was, Goku played with Zamasu and gave him a chance to walk away. Remember, this same fanbase got on Goku's ass for playing with Black when they first met despite Black being noticeably weaker than Goku and the fought only stalled after Black got sucked through the portal again. Goku also never offered any of them the chance to just walk away. Even before he learned about his family, he was trying to kill Black and Zamasu to the point of blowing up Zamasu's head.
Again, this isn't really related to Goku being dumb, Imo, but fans put this in the Goku is stupid column.
They took turns since after Vegeta refused to fused, Goku went to fight Merged Zamasu, while Vegeta waited literally waited on the side. This is after Merged Zamasu smashed them, making this scene even dumber. It become even dumber since Goku could have used Mastered Super Saiyan God Super Saiyan at that point, especially since he didn't know he only had to run the clock, so him going all out would make sense. Also, both of them saying no to fusion against a fused being who wasted them, is against both of Goku and Vegeta's characters shown in the Buu Saga. Goku only didn't fused against Kid Buu since he wasn't a fusion himself anymore. Against Super Buu Gohan absorbed, Goku not only wanted to fused, he begged Vegeta to see reason. So him just giving up when Vegeta said no to fusion is OOC for him.
Long story short, my point was that Manga Goku has more than one 'dumb' moment and fans are not just taking the sins of Anime Goku and putting them into Manga Goku, most of them anyway. Most of the problems people have with Anime Goku are in the Super manga, to a greater extent in some cases. Although, I will say that Anime Goku has more out there moments, mostly for comedy sake.
Now, I don't personally agree with most of them, except Goku and Vegeta took Black. Future Zamasu, and Merged Zamasu far more lightly than their anime counterparts, who regularly got blasted for not being serious. But I blamed that mostly on Black, Future Zamasu, and Merged Zamasu being such jokes in the manga, although Goku and Vegeta not working together and not fusing in the manga really was dumb and not in-character for either of them giving their development.