Hello, Is becoming drunk a sin (according to Rambam it seems so)? If yes, is it a Torah prohibition or a rabbinical one?

Although, you question is straightforward and simple, the answer is somewhat complex. On the one hand, we have many mitzvoth which involve the drinking of wine. Kiddush on Shabbat, 4 cups on pessach, and the drinking of wine at a brit. The amount of wine which of a "revi'it" which allows you to fulfill the mitzvah of Kiddush is the same amount which then prevents you from tefilla. (Shulchan Aruch Orach chaim 99), prevents a cohen from doing Birkat cohanim (Shulchan Aruch Orach chaim 128:38), and a Rabbi from giving a halachic ruling. (Shulchan Aruch Yoreh De'ah 242:13.) Therefore, we cannot say it is forbidden from the torah or even a rabbinic prohibition to drink and to become intoxicated. On the other hand, being in a state of drunkenness is something which is forbidden ethically. Our Torah is against a state of idleness and material indulgence, even before we get to the negative effects of being in such a state in regard to lewdness and the image one creates of oneself. In The famous book of ethics "Orchot Tzaddikim" whose author is unknown he quotes the Rambam as to say that an assembly of drunk people should be worse in your eyes than an assembly of people in a state of nudity." (Orchot Tzaddikim Sha'ar Hasimcha.) We don't have this quote of the Rambam in his writings that we have in our possession today but the Rambam speaks similarly ih Hiclchot de'eot 5:1 and Moreh Nevuchim 3:8. To sum up, I would say that it is permitted to drink however it is forbidden to remain in a state of drunkenness because we cannot properly serve Hashem through Tefila or Torah learning and it can lead to severe moral aberrations. All the best.