What does it mean when the reviewer says "the results are rather straightforward"?
Offhand, it could mean two things:
- The results are to be expected, i.e. not surprising. Normally not a good comment to receive since it implies the paper is not interesting.
- The results are gotten in straightforward fashion. For example, if you were asked to take the derivative of an elementary function, there's a well-known series of rules that will lead to the derivative. If the function is complicated enough then taking the derivative will also be complicated - but it is straightforward. Apply the rules correctly and you will get there.
The rest of the sentence leads me to suspect the second interpretation is correct.
If the second interpretation is indeed correct, it wouldn't be something to worry about, and your attention is better focused on what the rest of the review says.
When we do research we expect to find something new, preferably useful. When the results of an experiment are "straightforward" it means that the novelty is limited. One would have expected such results even without doing the experiment.
Since the novelty is limited it is possibly useful only to some specialists working in a niche area.
I would say, the reviewer is not highly impressed with your work. But at the same time does not think it is useless.