Even if you pick up your takeout food order, it's always a good idea to tip the server. After all, they have to take the time out for their other duties to prepare, pack, and serve your food to you. It's up to you whether you should tip as much as you would for a delivery order, however.
If you're watching your weight, you'll have to be careful with the takeout food. Limit your consumption of burgers and French fries, for example, and steer yourself toward low-fat meats like chicken. If that chicken is deep-fried, though, your best move is to peel the skin off before you eat.