Beckett may be an ensign but that does not mean she is inexperienced. She has served over 14 years in Starfleet; she has intentionally kept herself demoted to stay at ensign because she feels she makes the biggest difference on the Lower Decks. She has a bloodbond with a Klingon General, she has Riker hook her up with contraband, she is feared by Quark for her ability to navigate Ferengi economic theory and blackmail him, has friends ranging from outlaws, to Ferenginar to Kronos, and among her best friends are an Orion and a Vulcan. She initiated first contact with Galordonians which was a success as by the time of Lower Decks they are petitioning for Federation Membership and all of this was before the start of the Lower Decks series. She helps civilizations navigate and avoid Federation bureaucracy to help people. She then formed a bond with *another* Klingon on a death world where she was pitted against him in mortal combat and then convinces the nonhuman races of Nova Fleet to break away from Nick Lorcano's plan. She helped diplomatic relations between sentient cubes and sentient spheres while mentoring the grandchild of an alien who long ago masqueraded as Zues on Earth.
Xenodiplomacy is something Mariner has in spades.