There are a couple of factors at work here. First of all the basic answer is no, there shouldn’t necessarily be a reason that a cell booster wouldn’t work as well if you’re really close to it. But there are some reasons and the real world tends to complicate the way we look at things like radio transmission.
If proximity to the booster were a real concern, there wouldn’t be cradle devices like the weBoost Drive where the booster were actually touching the phone. If it weren’t possible to do that, well, weBoost wouldn’t do it. That specific booster is designed with low power and with a broadcast design that takes into account the wavelength of the signals.
Speaking of wavelength… that’s one reason people think that you can’t be too close to the booster. They think if you’re too close then you somehow won’t get a full wave because the distance between you and the booster is smaller then the length of a single wave. The most important thing to realize here is depending on which band you’re using, the wavelength of a cell signal is between 6 and 15 inches. So if you’re more than a foot away you are certainly not going to be concerned with that.
It is true that the antennas on cell boosters are going to be more effective from a distance because really close up they could potentially have dead spots. So basically if you put your phone right underneath the booster it could get poorer reception. But on the other hand the whole area is flooded with signal so you’re still getting a lot of boost when you’re close, unless you’re completely shielded from the antenna.
One real concern, in fact the only real concern, is putting yourself (not your phone) too close to the booster. It’s still not clear what long-term exposure to high levels of RF will do to you, and the signals coming from a booster are quite a bit stronger that those from a phone. We’re still talking about a fraction of a watt, or about 1/10th the power of an average nightlight, but there could be a concern there. I’m certainly not advising anyone to strap a booster to their head and walk around with it.