The issues is they NEVER gave her a reason to be OPPOSED to going ... and then the Brake Up occurred largely because Marco couldn't separate himself from Mewni. If the ONLY thing that was keeping him on Earth for the second half of summer, why didn't he just ask Jackie to come with him to Mewni? It's almost as if the thought never once occurred to him.

This either means either the writers or Marco were artificially keeping the Mewni and Jackie separate from one another ... and personally I prefer the latter choice, because it could result in Jackie's character being saved from the garbage and actually be a spot of really decent writing.

If Marco NEVER overcame his "Image of Jackie" (which from what we've seen on screen, he hadn't) and if that Image was of a "nice, NORMAL, Earth girl" then it makes sense why Jackie's never been to Mewni ... Marco was subconsciously keeping them separate because Jackie would represent one of the last few parts of his Earth life he had left.

I