My sense is the faux-outrage in relation to the carriage ride comes from two principal categories of people:
Firstly, there are the general "moaners", who will complain about pretty much anything. It could be a royal jubilee, the Trooping or a wedding, it does not matter. The arguments for and against the event in question do not matter, they will be complaining.
The second is a new category of small, but very rabid Meghan supporters, who will talk this wedding down, come what may. Not sure how the logic works as they are not, IMO, in any way "competing" royals, but I see a lot of pro-Meghan posters being very negative on this wedding and the carriage ride. Perhaps it is just an unhappy coincidence.