And those "opinions can be critical too if people so wish. Or should we all have to have the same opinion and say how wonderful it all is*
Funny thing is, I have never once made any suggestion that people should not have their own opinions, so I'm not sure why you are asking that question.
What makes you think people are unhappy and that they are "revelling" in bringing someone down
Comprehension skills and life experience. People content with their life generally look at a wedding such as Harry and Meghan's, which is clearly a love match and appreciate the good stuff. Unhappy people tend more to want to find fault, often to make up for their own sense of inadequacy. (It's often not justified, but people feel how they feel). Of course, it's not an absolute rule, but it's definitely a 'thing'.
And no, I'm not saying I'm perfect. I know that I have the same tendencies myself.
Not everyone loved the dress, no harm in people saying so and the reasons why
No, no harm in yet again women being fair game for criticism. Except there is. The whole acceptability of making women fair game for judgement on how they look is very damaging. And it's everywhere.