That's a bad comparison though. What you're describing is a ROLE that both characters (movie Gwen and Ultimate MJ) served, not a TRAIT. Their role is faithful girlfriend/partner/lover. Doesn't have anything to do with either of their characters and personalities. It's like saying ASM2 Electro is comparable to comic Doctor Ock because they were both Spidey villains and tried to destroy New York. Those are roles that different characters can serve as no matter what personalities they are.
And like I said on another thread, Ultimate MJ ISN'T EVEN MJ. She's basically the "girl next door" part of 616 MJ extracted and turn into a stereotypical "girl next door". Her personality is completely different, she's more of a geek and fits more of the "good girl" archetype that the original Gwen played. So... 616 Gwen --> Ultimate MJ --> movie Gwen... Nothing really wrong there.