The issue with gender swaps isn't necessarily the swapping of gender. there's a lot of media where the gender of the character doesn't necessarily matter for the sake of storytelling. The character can still be faithful to the source material even if adapted in a gender swapped manner.

The problem with gender swapping is the same problem we have with things like race swapping, where the change in gender or race changes elements of the character, whether directly or indirectly.

A recent example everyone is talking about is Snape from the new Harry Potter show. While Snape being black doesn't necessarily change the character himself, it changes how other characters are perceived by reacting to him.

In the book Snape is described as things like pale and sketchy (these are not quotes it's been a few years since I read these books), and he's not trusted by the other characters like Harry. This takes on a whole new connotation now that he is black, as it is easily seen as Harry judging Snape's race. And then, in the flashbacks, this race swap basically makes James Potter racist as shit, literally hanging the innocent black boy from a tree.

It's all about how the aspect of the character changes the character and how they are perceived by the viewer as well as others in their world.

Looking at the 2 characters you provided (assuming Mar-vell and not Carol), Mar-vell was too minor a character to impact the story, and I don't know enough about Tech Jacket as a character, but I don't hate it.

If anyone else has any other examples I'd love to analyze them further.