I think discussing whether Jayce and Mel might be doomed by the narrative is valid because of the nature of the show. It's a show that likes to hurt us, after all (I think they should end up together for numerous reasons but I can't predict the future).
What's not valid at all is to insist that they do not honestly love each other or that they have not consistently been a compassionate and healthy pairing. In fact, I feel like their personal flaws kind of dissolve when they're alone with one another and they become the kindest and most empathetic versions of themselves.
I think it's valid if the ship is not your personal taste and you'd prefer something different for them. Not everyone is going to like everything that you like, different strokes for different folks.
It's 1000 percent not valid to claim that Jayce is just using her because he can't have Viktor, who he truly loves but can't be with for whatever reason. It's not valid to say that Jayce has been a terrible friend who is never there for Viktor. It's not valid to say that Mel is (still!) just manipulating Jayce into doing her bidding with her magical coochie and she isn't capable of having true feelings for him. In fact, that first thing and especially that last thing are misogynoir, don't do that.
It's not that you don't like the ship, it's that you can't stop projecting your own feelings onto them and pretending they're facts. I really hope this helps.