If he/she has that shape only around another Molly of the opposite sex, it's part of the courting dance they do (you'll see Platties and Swordtails doing that too)..if it's a permanent thing, then you've got problems, a possible infection..as to what, I'll let one of the other members with more experience in fish-diseases weigh in.
Uhmm I hate to say this, but I had 6/16 baby fry like that..
and well after about a week they ..died..
I find that dalmation mollies have quite a few problems usually
(or maybe it's just me) they're beautiful fish.
but definetly NOT recommended for beginners.