I don’t think the ride is particularly bad when compared to Peter Pan or Small World or other similar format dark rides. Sure there is no nostalgia attached to it but when you look at the intended audience for the attraction it fits in well within Fantasyland.
I think people forget that it is also strategic, as it adds vast capacity to the park without significantly boosting attendance. This is significant because the park needs rides which can handle a decent throughput but tend not to bring more guests to the park specifically for the attraction. 7DMT is an example of what happens when you get this in reverse - it’s attractiveness is far higher than its capacity can handle. Too many rides like 7DMT and you would end up with a hot mess.
Sure, It’s no Mansion or Pirates but Mermaid was never meant to be that highly detailed format - it would have ended up increasing attendance too much if the ride was out of this world fantastic.