It's not like her mum hates her. She's giving good advice. Those fatties who under-dress look disgusting and pathetic the fit girls make fun of them and they're not going to attract anyone, unless it's some dude looking for a pick-me-up, any-hole-will-do, one night stand.  So either (1) she'll continue to be fat and ignore it and continue to be miserable and unhealthy but at least not make a fool out of her self or (2) just maybe she'll be motivated to do something about it, thus, living longer and be genuinely happy.