In short, no, stickers will not ruin your car. These stickers are made with specific products made for cars, which means that the product should easily slide off. The best way to get these stickers off without damage is to use warm soapy water and either gently pick it off, try to roll it off, or use a scraper - but if you do use a scraper then make sure to be extra gentle and careful as to not leave marks on your car.
Once you have got the sticker off then clean the area with more soapy water to get any sort of sticky residue/ left overs off.