well maybe if you didn't reject him peach he would be with you instead of bowsette.
And maybe if Mario had proposed at a sane time and actually proposed romantically and focused on her, instead of focusing on one-upping Bowser more than on her and doing it when she was in a bad mood from being kidnapped, she'd have seriously considered it instead of being pissed off at the rudeness.
Which raises the question of why she even cares? If she rejected him, she shouldn't care. (And the original Bowsette image has her more just weirded out that Bowser just genderflipped than being outraged at Mario for not being with her, since she didn't want him.)
Like my mom. When her boyfriends care about her, she could give a rats ass about them. If they stopped caring or straight out ditched her, ooh we would not hear the end of it.