In terms of a sweet-spot for guests, a walk-on is a positive, but Disney probably perfers at least a 10-15 minute line, time to enjoy the queue and get some of the masses off the walk-ways. For Mermaid in MK, you can bet that they want that interactive queue filled with 20-30 minute wait time lines, taking warm bodies off the walkways (and out of other lines for the popular attractions). If Mermaid is a walk-on running at 50% capacity, it won't do much to decrease the crowds at all.
Well, I would estimate that DCA's Mermaid runs on about 50% capacity, much less when I boarded the clam shell as there was nobody else in line. Haunted Mansion has a long queue, but the doombuggies practically *all* get loaded with a 30 minute wait. When HM is a walk-on, you go straight to the elevator and you go down and get on a doombuggy while some go-by empty, hence no bottle neck per se. Guests love going on HM again and again, no so much with Mermaid as it doesn't have a lot of details, and there are a lot of weird stylistic choices.
I wouldn't rate Mermaid as being on the same level of Spaceship Earth, or even the old World of Motion. It's got 3 good animatronics, and everything else they sort of went cheap-o on, especially the second-half. Maybe MK's Mermaid queue will make the difference . . . but the ride just didn't seem like it was designed very well, they just lifted some scenes from the film, plopped in some screens and duplicate fish and that was it. Technically, Enchated Storytime with Belle has the best animatronic in MK at this point.