@Chrono
The original text, in Japanese, about Bepo and Zoro appearing in that cover of chapter 1031 makes it a lot more clear that Oda did not choose the characters based on strength, especially because of the use of 勝手に, which gives a strong impression of thoughtlessness in the decision to choose those No.2s;
And there's really a false translation in the text provided by EtenBoby: Oda didn't say that he "picked out" them; Oda said that he 'picked up' them, and there's a relevant difference in meaning between these two, because "pick out" gives the impression of distinguishing them from the rest, and that's not what Oda did; Oda just picked up (ピックアップした) those characters.
There's no indication in the text that he chose them based on strength, so it seems a lot more likely that Oda just chose the characters based on how cool they would look together in the cover.
Also, what Oda wrote about Bepo's Sulong form actually does the opposite of confirming that Bepo was chosen for being the strongest of the three;
Oda wrote that in normal circumstances Shachi and Penguin are more reliable, but due to the fact that they saw Bepo's Sulong transformation, they recognize that in terms of strength in battle (戦力的に) they can't match Bepo.
This is not an acknowledgement that they are really overall weaker than Bepo as combatants; It just means that they can't reach the same level of strength that Bepo has while he is in his Sulong form, which is not relevant considering how they are stronger than Bepo during all the rest of the time.
The Mink's can only use their Sulong transformation at night, and only when there's a full moon for them to look at, which doesn't happen every night, so Bepo's powerful weapon that allows him to become stronger than Shachi and Penguin is a very situational thing, and definitely not some kind of superiority for one to brag about.
During by far most of the time both Shachi and Penguin are stronger than Bepo, so they are the ones who are overall stronger than Bepo, and clearly not the other way around.
And this is a clear proof that being drawn in that chapter cover did not necessarily mean being the second strongest of the crew.
Source:
https://pbs.twimg.com/media/FPPQInmWUAobp8y?format=jpg&name=large