Does the use by girls of masculine forms for adjectives (such as: jāʾiʿ instead of jāʾiʿa), and the word "yazm" among them, fall under the category of imitating men, taking into account the prevalence of this habit and its not being considered strange in some areas?

Using a common local dialect word, not intending to imitate, is not considered a forbidden imitation. Such instances are found in many Arabic dialects. Arabs may also drop the feminine "taa'" (ة) from adjectives describing women, so one might say: "Imra'at 'ashiq" (a loving woman), "Imra'at baligh" (a mature woman), and "Imra'at khadim" (a serving woman).