I've been following this quite closely and, unless Ford is carefully throttling deliveries to reservation holders to maintain appearances of strong demand (which is certainly possible), the slowing sales are due to supply in the US, not demand. Because it's taking them a long time to fulfill pre-orders and many are still not filled yet. But it's also obvious that demand is not huge (the Mach-e is a very expensive car for what you get) so it's possible this is just Ford playing games to keep perceived demand high. There is at least one large shipment of cars that's been sitting in a European port for well over a month and yet they haven't sold a single Mach-e in the EU as of the end of Q1. I think they are waiting for software updates.
The roll-out of the Mach-e has been a bigger boondoggle than the Model 3 ramp and yet the media is strangely silent on this issue. It's almost as if they are still hoping the Mach-e might still be a threat to Tesla.
My impression is that a fair number of US Mach-e reservationists are starting to get buyers remorse and many that have not taken delivery are backing out of their reservations. Many bought it primarily because they wanted an EV but hated Tesla or they liked the Mach-e styling better than the Model Y. Now they are realizing Ford over-promised and under-delivered. Phone as a key is not even close to as functional as it was this time of the rollout of Model 3 (most admit they don't even use the feature because it's so unreliable), the Ford phone app is terribly flakey, Plug and Charge almost never works so they can't use the free miles Ford gave them, the software in the car is non-sensical in many ways, the remaining range metric is completely unreliable because it works like it does in Ford cars and Trucks, it adjusts the remaining range way higher than reality if you have been descending out of the mountains and adjusts it scarily low if you just climbed a mountain pass such that it can never be relied upon, even by a reasonably experienced operator. There is no way to convert the percent of battery charge remaining into miles without doing the math in your head or using this inaccurate "remaining miles" display. A large percentage of the cars have glass roof panels that are misaligned with the top edge of the windscreen such that it makes excessive wind noise and I don't think Ford is considering this a warranty defect. Most of the center touchscreens make a buzzing noise that is irritating to most when the car is parked. The suspension is tuned in an odd way that causes it to feel bouncy on typical highways above 50 mph such that the headlights will be fluctuating up and down while other cars on the same road have steady beams. You can't use the Ford charging cord on a 240V 30 amp outlet in your garage. Even with an adapter the car will try to pull too many amps and trip the breaker (there is no manual way to reduce charge current in the car).
The list goes on.
This is really too bad for the transition to sustainable transport because I think it will cause some of the Mach-e buyers to be turned off from EV's for future. purchases. I know it's difficult, but Tesla owners need to resist the temptation from strengthening the "us vs. them" tribe mentality by acting like superior douchebags. Just support their decision to go electric and tell them it doesn't matter which EV an individual gets, as long as they are happy with it.