Is the maternal uncle of my father considered a Mahram (unmarriageable relative) to me?
Yes, he is, because you are one of his sister's daughters, no matter how the line of descent goes. You are included in the ones mentioned in Allah's (Exalted be He) Saying,
Forbidden to you (for marriage) are: your mothers
to His saying,
...your sister’s daughters
which means no matter how the line of descent goes. (Part No. 17; Page No. 388) May Allah grant us success. May peace and blessings of Allah be upon our Prophet Muhammad, his family and Companions!