If the cashier is using your first name and you're not a regular who they're familiar with, nine times out of ten it's because it's company policy since in their view it fosters a friendlier environment for the customer. Now, whether that's true or not depends on the individual customer's perspective, but regardless, it's highly unlikely that the cashier decided to do this on their own, so it's best not to blame them for it.
Plus, I think creepy is too strong a word. There's no mystery where they got your name from, since it's right on your store card, credit/debit card, or check. If you had never said your name before, never used a store card, and always pay cash and they still knew your name, that
would be creepy. This is just being a bit overly familiar to make you feel welcome at worst, and the issue is most likely at the company level, not with the employee.
Now, as far as commenting on your purchases, if it's just saying "Oh, that juice tastes great" or something harmless like that, it's just making small talk and trying to be friendly or break the monotony, as said above. If they comment on personal hygiene products and make you uncomfortable, say "Wow, that's a lot of liquor" and make you feel like a lush, or make a joke about the large size of the clothes you're buying, that would be inappropriate. Otherwise, what's the big deal? If you don't like it, it's at most a couple seconds of grunting an affirmative and nodding in response to treat the cashier like a human being; hardly any great hardship.
It's really a thankless job. Because you're the last employee the customer usually encounters, you get the sum total of their complaints if they have any, most of the time about shit that's way above your pay grade or which you have little or no control over. You're required to interact with people even if the customer is having a bad day and just wants to tell the world to fuck off, and that doesn't even take into account how you're feeling. You have to deal with people doing stupid or time wasting shit all day long, with the full knowledge that for every second they waste, you're probably going to be the one who gets blamed for it by other customers unless the stupidity is obvious to everyone. It's mind-numbing monotony coupled with back and leg straining physical work and standing in place (which is worse than being able to walk around). Plus, it combines everyone's favorite activities of having to stand in lines, being herded through small lanes like cattle, being in extremely close proximity to others, making small talk with strangers, and then having to pay for the privilege. YAY!
So knowing all that, unless a cashier is really a horrendous douchebag, I go out of my way to try and be as nice to them as possible and to make things easier for them: bagging things myself if need be, making sure my method of payment is ready to go before I get to the checkout, being polite and responding to their questions/comments, organizing my stuff by category so it's easier to bag, etc. I'm not saying everyone should have to do this, because it is the cashier's job, but that's just what I try and do because of past experience. I worked as a cashier when I was in high school and while most of the time it's pleasant and you get to interact with nice people, it can be an awful, soul crushing experience at times.