At the end of the day, it was still a quick teaser to show off Ariel. It served its purpose as a teaser rather than it being a full on trailer.The snippet of the song that they included in the trailer was NOT what they should have included. It was an awkward part of the song, where for the casual viewer (probably most of the dislikes) they would have needed to hear the build up to that part to appreciate it - otherwise, it sounds off and different from the original.
Honestly, if I were DISNEY, I would have shown "Under The Sea" to show off more of what the film will actually look like (the actual sea animals like Flounder and Sebastian) and also give the trailer some life with an upbeat song. They took a major risk showing off the part of the movie that will be the most scrutinized - "Part of Your World" which wasn't the best idea IMO. They should have left that to be something for people to look forward to in the film, but now it's already given people a bad taste in their mouths because of the poor choice of audio clip that they used. Oh and I would have included like a shot of Ursula - even if it was just tentacles or something.
But very clearly the youtube dislikes is because of racists rather than anything to do with the teaser content itself.