Is copying lab results a form of plagiarism?
Plagiarism is typically defined in terms of taking the work of another person and passing it off as your own (see, e.g., the Wikipedia page).  It does not matter whether the material that you steal is words, data, pictures, etc.

Thus, I would classify the practice of copying lab results that you describe as plagiarism.  It could also be categorized as other types of academic dishonesty as well, such as falsification of experimental results.