The fact that unlike some other characters, he's actually trying to change, and improve himself, makes the constant barrage of hatred against him, just so damn annoying.
I just wanted to see people talk about Duet, the musical episode, which was awesome, and people seem to be constantly dragging on Mon El, and because of it, on the episode as well.
It just feels like a repeat of how people acted about Riley Finn back in s4 of Buffy. it was enough to start the seeds for having me grow out of Buffy/Angel then, so you can imagine what it could lead to if it keeps up.
I'd get it if people just complained about the character on the anti tags, but they keep insisting on doing so in any tag even remotely connected to Supergirl, and worst of all, in the Mon el tag and karamel tag as well.