It's funny because Shimerman blames himself for the Ferengi being failed villains in TNG. He saw DS9 as his second chance to make things right. It's true the Ferengi never really became "the new Klingons" in terms of overall threat, but Quark certainly had his moments as a criminal. I also think it pays that the Ferengi on DS9 aren't Maurader crew, they're just civilians. Might as well be shocked that the Romulan soup seller lady in TNG's Reunification wasn't as capable as the many Romulan soldiers we've seen the cast deal with.
Honestly I think it was a much better direction for the Ferengi. They just aren't very physically imposing, so ranging from being nasty little buggers to bumbling comic relief isn't really a bad place for them. They might not get respect but you underestimate them to your own loss of profit, and you certainly don't want to turn your back on one.
Speaking of bumbling comic relief this interview reminds me of just how wild Rom's personal ride was. It's hard to compare his later kind, genial, decent, lovable goof version to the early asshole who was more than happy to attempt flushing his own brother out an airlock. About the only way I can think of to reconcile the two is that early Rom was overcompensating, trying to be the quintessential Ferengi because that's what his brother expected, but dealing with the Federation gave him a different way of looking at things and new opportunities to be his (personal) best self. Basically Quark and Garak's root beer theory of the Federation. It was definitely good for him but I still can't see him as the Grand Nagus. Hardliner rich bastard Ferengi will send hired assassins to snuff him out within a week.