Why was moses dressed up as a mitsri when the daughters of yisro found him and according to some midrash he had left Egypt 50 or more years previously. Being Jewish it is unlikely he spoke like a mitsri. Since the Torah stresses this fact there must be some reason for it apart from him being an Egyptian prince and since he was running away from pharaoh after having a death sentence, he most likely lost that 'position'. And why did the Jewish people on leaving Egypt specially want the 'smolos' the Egyptian clothes which rashi says they even preferred to the gold and silver. In Egypt they never wore them as it says they never changed their names speech or clothes. They most likely wore Egyptian linen even in Egypt only their fashion styles were different. So why change their 'fashion' on leaving Egypt.
Moses was dressed as an Egyptian because until he fled he was an Egyptian prince! Though the purpose of the pasuk (and midrash) stressing his Egyptian identity is indeed a complex one. The text wavers back and forth whether Moshe's identity was primarily Israelite or Egyptian.
The Mechilta does not mention clothing:
though clothing does appear here:
Based on this midrash, clothing was important as identifying mark of Israelites as a separate nation.
Regarding borrowing Egyptian clothing on leaving Egypt, indeed Rashi (Shemot 12:35) says this, citing Mechilta. As Gur Aryeh (quite plausibly IMHO) explains it, this is that the Egyptians, not the Israelites, were the ones who held this clothing particularly dear.
I do not think that these midrashim, from different sources -- Mechilta does not specify clothing earlier as identifying Israelites, and is the source for someone holding clothing dear -- are supposed to be combined to make a nice and entertaining devar Torah, though surely there are people who have constructed such divrei Torah.