Translation:The Mexican restaurant is very close.
I suspect that it's the station for trains. Trains is a noun, and maybe Spanish isn't happy with using nouns directly as adjectives like English does. Mexicano is already an adjective, so doesn't need any help. Notice that you also have to use de with descriptions like ensalada de tomate, but not ensalada verde.
The 'temporary/permanent' thing is not a very good metric and this is exactly the reason why. We are talking location here. Please reference the following... take special note of the Mantra.
Mantra: For how you feel and where you are always use the verb Estar
Estar is used to indicate transitory states and locations. If that general rule doesn’t suffice, there are two acronyms that you can think of, PLACE and LoCo. PLACE stands for Position, Location, Action, Condition, and Emotion. LoCo stands for Locations and Conditions.
Ser is used to talk about intrinsic or lasting attributes. If this general rule is too vague for you, think of the acronym DOCTOR, which stands for Descriptions, Occupations, Characteristics, Time, Origin, and Relationships.
When it's used for distance, there's usually an implication that it's quick or easy to get to for some purpose. Like "We don't have much time, so we should eat at the Mexican restaurant, because the Mexican restaurant is very close." Or "The fish dishes at the Mexican restaurant are very good, because the fish pier is very close."