Serious Mozart is definitely a thing, and Kuijken is one of the people who explored it most. His Mozart quartets are like that, as are the Mozart violin sonatas with Leonhardt and Luc Devos. There are people who have followed him, Saloman Quartet for example, and Ryo Terakado in his Mozart Sonatas with Boyan Bodenitcharov. It's almost a tradition!
Also note that there are pieces by Mozart which everyone plays seriously, for example the Masonic Funeral Music, the Fantasy and Fugue in F minor K 594, the C minor Adagio and Fugue, the Fantasy K475. So it's not as if this way of playing at least some of the chamber music is wrong headed, rather than just something Andrei didn't enjoy.