I don't see a reason why one couldn't make a siyum on a masechet of Yerushalmi but I was wondering if there are any halachic sources that discourage it or address this specific question.
Rav Aryeh Tzvi Frumer writes (Eretz Tzvi 2:74) that technically a siyum is made on a masachet of mishna (as is clear from Shabbos 118a that in the gemara's time they made siyumin before chasimas hatalmud). Therefore, he explains, one can make a siyum even if some of the Talmudic text has been censored and one is not learning the entire masaches in Gemara.
Thus, learning Yerushalmi is also learning the maseches of Mishna albeit with a different peirush.
The reason we don't make a siyum on every maseches of mishna but rather require completion of a whole seder of Mishna (according to many Poskim) is because it is not a significant enough accomplishment for a siyum celebration.
(If for a particular individual it is a significant accomplishment-- e.g. for beginners in learning -- it may be sufficient for a siyum as is evidenced from the famous story with Rav Moshe Feinstein and the beginner in learning who, after completing one daf, was encouraged to make a siyum.)
For a larger discussion see here.
Rav Moshe Feinstein in Iggros Moshe O.C. 1:157 writes that the basis for making a siyum is from the Rashbam in Bava Basra 121a who explains the reason for the simcha on Tu B'av is because of the completion of a mitzva which extended for a long period of time, and the Nemukei Yosef there states explicitly that there is a basis to make a seudas mitzva upon the completion of a mitzva which took a long time. If that is the case, finishing a tractate of Yerushalmi should be no exception.