I wouldn't get it. There is a lot of pressure to lose weight and it sounds like your MIL has internalised that pressure and equates losing weight with happiness and fame. I think a gift should be a "I like you right now", rather than "Here, become a better person".
The kind of attitude your MIL has towards losing weight is not a healthy one. She's fallen prey to the media pushing the idea upon her that with thinness comes automatic happiness and success. Which is far from a healthy mental place to be in when facing body issues of any sort.
I think perhaps, unless explicitly asked, it's best to stay away gifting anything that implies a change is desired, expected, or needed of an individuals body.
If someone said "Yes I'd like some hand weights for the Holidays" you are explicitly being told that this item is so desired and would not cross the boundaries of that individuals personal being.
Or if she said "I really want to join the gym, I'd love a membership there."
Someone saying "I am struggling losing weight. Boy I wish I was thin so I'd be happy" is not an offering into crossing that boundary and is not an attitude you should be supporting in the first place.
If you are dead set on providing something you see as being supportive of her body-changing goals perhaps a non-weight related item would be more appropriate. Say, if you know she wants to start making energy smoothies, protein drinks and other such supplemental drinks you could purchase her a blender? They sell blenders that are small but carry something along the lines up 16OZ and the 'cup' part of the blender is made to be detached and carried like a mug with a screw on lid (some have straws that are thick plastic and made to be washed with the mug).
This particular item is not implying anything of how you view her or her body or any expectations you might have of her. Nor is it an item specific to weight-loss.
Adding into it she's openly discussed having trouble staying on 'track' with her goals buying her a $100 tech system may simply end up being unused and collect dust.