It is definitely NOT your imagination. Mariachi trumpet players have some very distinctive characteristics s in their playing; in fact, there is a "Mariachi school" of trumpet playing. This style is studied by those players just as the classical and jazz styles are; although, in fairness, there is arguably less complexity (if not subtlety) in that style.
The most distinctive aspect of this style is the use of very fast, but very wide, vibrato; much wider than than used by most jazz or classical players. There is also a very unique and "punchy" way of articulating notes. Articulation is the way that the note is started, usually by the use of the tongue. Mariachi trumpet usually "hits" the note with the tongue in a more aggressive way than in jazz; and certainly more than in classical, where a greater level of finesse in articulation is appropriate. Notice that I said "appropriate"; this lesser level of finesse is not necessarily a deficiency on the part of the players' technique, and is used to fit the style. Interestingly, studies have been done (and this has relevance to audio) that show that recorded players' tones (any instrument) are very difficult to distinguish from each other if you remove the initial attack of the note (articulation/"leading edge" in audio).
More subtle points are preference for a brighter/brassier tone than in other styles, and a tendency to play on the high side of the pitch. Just as with rhythm where some players play on the front or back side of the beat, some players play a little on the sharp side of perfect intonation as opposed to on the low side of the pitch (many jazz players). In jazz, playing on the low side of the pitch can highlight a "bluesy" sound. In Mariachi, playing on the high side of pitch gives the sound extra brilliance. These can be subtle deviations from perfect intonation and not necessarily identified as being out of tune.
Viva Zapata!
The most distinctive aspect of this style is the use of very fast, but very wide, vibrato; much wider than than used by most jazz or classical players. There is also a very unique and "punchy" way of articulating notes. Articulation is the way that the note is started, usually by the use of the tongue. Mariachi trumpet usually "hits" the note with the tongue in a more aggressive way than in jazz; and certainly more than in classical, where a greater level of finesse in articulation is appropriate. Notice that I said "appropriate"; this lesser level of finesse is not necessarily a deficiency on the part of the players' technique, and is used to fit the style. Interestingly, studies have been done (and this has relevance to audio) that show that recorded players' tones (any instrument) are very difficult to distinguish from each other if you remove the initial attack of the note (articulation/"leading edge" in audio).
More subtle points are preference for a brighter/brassier tone than in other styles, and a tendency to play on the high side of the pitch. Just as with rhythm where some players play on the front or back side of the beat, some players play a little on the sharp side of perfect intonation as opposed to on the low side of the pitch (many jazz players). In jazz, playing on the low side of the pitch can highlight a "bluesy" sound. In Mariachi, playing on the high side of pitch gives the sound extra brilliance. These can be subtle deviations from perfect intonation and not necessarily identified as being out of tune.
Viva Zapata!