Could be. I think him being mentally weakened is primarily why he was overwhelmed by Gotenks. Whether that be his battle power dropping, fighting sloppily or him wielding his power poorly, I think the end result remains the same.
Piccolo says that Gotenks was at least as strong as Buu, at the least, Gotenks and Buu are equals, if not, Gotenks has the edge. I mean, as you use Super Buu as example, I can also use him as well, he couldn't grasp SSJ Vegetto correctly neither realize that Vegetto was stronger than him until too much time after, the same can be applied/inferred with SSJ3 Gotenks.
Even though, Trunks is arrogant, in this case his case is credible at some point because he is proving himself right, Goten never says Gohan is much stronger than Gotenks either, he just said he was stronger.
I honestly don't really have much to say. Your interpretation really is valid. My understanding is that Piccolo wasn't sure if Gotenks was stronger than Super Buu hence why he suggests that he's at least "on par" with Super Buu. That leads me to believe that Piccolo's uncertainty would suggest is if there was a gap between the two, it's not by much. Super Buu was overwhelmed primarily because he was mentally weakened IMO.
Even then, Goten's correction doesn't include Gohan being way stronger than him, just a little bit...
Super Buu also says Ultimate Gohan was only stronger than him.
Trunks agreed that Gohan was just a little bit stronger (which is actually quantifiable comparison)... Why was Buu disrespected by Goku while claiming that Gohan can defeat him, while Gohan didn't give Gotenks that treatment and actually was confident on Gotenks defeating Buu...?
Gohan never disagreed with Buu's recognition of Gohan merely being stronger. Regardless, Gohan told Gotenks to be careful so he wasn't certain of the outcome, unlike Goku who was certain that Gohan could have beaten Super Buu. False equivalence.
Because the fight was about to begin, on top of that, he was being praised by Buu so he wouldn't mind; when the comparison between Gotenks and Gohan was made they had more than enough time to discuss as Super Buu hadn't appeared.
I don't really doubt Gotenks' strength here. All I am saying is that this is a false equivalence because Gohan was never completely confident in Gotenks' chances of winning whereas Goku was confident in Gohan beating Buucollo.
It was actually given a fair comparison and that can be accurately understand, Gohan was just a little bit stronger, unlike Buu's vague statement; and for a good reason, both Gotenks & Gohan showed to be able to dominate Buu.
Super Buu already saw Super Gotenks in action, was mentally weakened by Gotenks displaying power that was "at least on par" with his own, felt the weight of Super Gotenks' blows, and even witnessed a fully charged attack by Super Gotenks. I think the narrative shows Super Buu had ample time to recognize Super Gotenks' strength and yet Buu only acknowledges Gohan as his superior.
Gotenks was trying to provocating him and infuriating him during the whole fight, on top of that, he was just serious at the end of the fight, which wouldn't be enough to accurately grasp or judge his capabilities, if Buu couldn't grasp neither it realizes that Vegetto was stronger than him, how is expected for him to grasp Gotenks properly? Gotenks & Vegetto were the only ones capable of making him mental unstable, similar to how Gohan did it with Cell. Gotenks was about to blow Buu into pieces, yet, he returned back to his base form, so at no point he used a full power blast.
Even though, in this case, Trunks is being proven right, he receives Gohan's supportment, as well as he previously had Goku's, he is proving himself right, regardless he is arrogant or not, there is no merit in which he can be rationally belittled, it's like doubting that SSJ2 Gohan was stronger than Perfect Cell in the Cell Games; it's something illogical.
