While there will be a niche for this type of game I don't see it being very popular.
Your starting off with overweight love interests as the primary and then you change them into more fit versions of themselves. This is also harder on the dev as your talking about changing the models look on the regular, between the work to do so and keeping track of each model that is a good amount of work without as much story progress.
Also mini-games that your constantly having to play is not a welcome thing in general. Flip it around where it's the primary game play and unlocks the story that might work out better but most people are not looking to play mini-games when trying to knock one out.
I don't think however the idea is not far off the mark however. Have the MC be a fitness instructor instead, this opens it up to a much wider variety of clientele from those who are over weight to those who just want to get toned and all the way to those who want to get into fitness / bodybuilding competitions.
Leaves it more open to peoples interests making it more likely to get supported.
Even still I do think it's a lot of work for a dev to have to regularly change multiple models.