What I can see is that this hypothesis is way too general for practical purposes and leaves several open questions. But you have already mentioned this in the answer.
Furthermore, I think that including the notion of observation might make sense in a case in which it this notion itself is of particular interest. However, it is usually necessary to specify the hypothesis.
We could look at different examples:
"One is more likely to observe a kangaroo at night than during the day".
If your goal is to take a photograph of a kangaroo, and want to decide when your chances are best this hypothesis is still better as:
"There are more kangaroos in the area of interest at night than during the day"
It might be the case, that you have a limited view at night, which leads to a situation in which there are many kangaroos, but they are hard to observe.
However, in this example it might be better to include the additional information in the hypothesis.
Next example, "Children are more likely to observe explicit content in TV at night than during the day".
Here, you might be interested in finding a good time in which children could be advised to watch TV. (Trying to minimize the likelihood of them observing explicit content)
This hypothesis is still better as: "There is more explicit content in TV at night than during the day". Just think of a case in which there is such content in TV, but children cannot access it.
To conclude, I would not say that it is generally a bad idea to use the notion of observation. Maybe think of cases in astronomy in which you are not only interested in specific things to happen, but also if we can with our limited capabilities will be able to actually observe phenomena.
However, in many cases the notion of observation may be something like a bad smell that might indicate that the hypothesis does not formulate exactly what you are interested in.