If John really had major influence inside Glendale regarding projects not tied to Pixar properties... then why wasn't he more involved at the beginning of TLM's development? Why was he fixing problems after opening instead of before? Why didn't the execs seek his advice before the criticisms started flowing from less-than-stellar reviews?
From what I gather, Lasseter didn't like the low budget that was given to Mermaid. No R&D was spent on a new ride system/vehicles, most the $$ went to some animatronics. Lasseter did wage a battle to keep funding up for Carsland, and he won. The fact that Lasseter was brought in to make changes to Mermaid after the fact, might well hint to rising influence.
It wasn't patently obvious from the start that Mermaid wouldn't be a big draw for DCA. I think the lesson from watching the guest reaction to DCA's Mermaid and Carsland's is that future additions should be part of a themed experience, versus trying to simply retro-fit an attraction into a land. DCA's Mermaid is sort of hidden, easy to miss, and just doesn't feel right in terms of where it is supposed to exist. Near the entrance of Mermaid you've got San Fran street, you can also see some of Grizzly River Run, in addition to Paradise Pier. If they had spent more money on Mermaid, by making a stellar ride, plus adding a D/C Ticket, and a restaurant/M&G it would have been something to write home about.
So, in essence, doing things the Lasseter-way won out. I don't think WDI's top brass, and more importantly top Disney brass are complete dullards, they know that Lasseter has a certain magic if given a big enough check, plus they've learned valuable lessons with Carsland, and have come full circle back to making quality attractions/lands for their theme parks.
I think something odd happened with Mermaid, especially given that Tony Baxter didn't have input on the project, which is very odd given that he conceptualized such a ride, though with a much different slant than the current one. Some "scenes" in DCA's Mermaid are amateurish and don't add to the ride, such as Ariel getting her legs back presented by a flatscreen tv, or Ariel kissing Eric and getting her voice back (not in the film), and also presented by a flatscreen tv. I think somebody tried to create a name for themselves by building a ride on the cheap that they thought would be a big success despite the low budget.