When I interviewed a couple of years ago, the mixer was at 6:30 p.m. (don't know if this has changed at all). A flight that arrives at 1:45 should leave you with more than enough time to get down to Galveston. Also, the mixer is super casual and has no bearing on admissions decisions; if you wound up arriving a little late due to travel, I seriously doubt anyone would care.
All that being said, if it's feasible for you to attend the night before social, I highly recommend it. It was really nice to get a feel for the UTMB community in a more relaxed setting than what you'll be working with on interview day.