speakers don't just emit sound towards the front, they emit sound in 360 degrees despite the efforts of designers. Higher frequency drivers are more directional than lower frequency drivers. The lower the frequency, the more 'omnidirectional' the device is.
By attempting to measure frequency response vertically above a speaker, you are going to experience interference generated by the reflection directly behind the device - i.e. the floor. In your scenario, the distance between the device and the floor is likely to be quite small (as I assume you are trying to avoid having to loft the mic and the device). If you imagine a distance of 1m to the floor, you will potentially experience interference in the 343Hz range.
Having a lofted device and measurement mic, you will likely have interference at a much lower frequency than if you were to attempt measurement vertically as you have suggested.