I don't understand the problem. In real life, artificial hearts exist. In BG3, robot technology is more advanced than in real life. Why would Karlachs heart be very silly then? And as a dramatic plot device, I think it works. Having a friend or lover who is destined to die soon is dramatic.
There are plenty of things that take the sillyness much, much further. People with lizard heads for example. Or invisibly carrying stuff that should require a big removal van. Nobody but your party using the big bright teleportations portals that are everywhere...
