Yes, it's a hard age. He's being challenged (puberty, hormones, peer pressure, figuring out this whole teenager thing), and so are you (your first baby growing up, hygiene issues, that disrespectful attitude and mouth).
I encourage you to get rid of the "meanest mom ever" mindset, even if it's just in your own mind and even if you've never said it aloud. I know you're being sarcastic, but still, replace that with positive thoughts, like you're providing for him, trying hard to be a good mother, and similar thoughts. If you've said that out loud to him, ask if you can talk to him, and apologize for those words. Tell him you said them, but in reality, you love him and you acknowledge that being 13 in a new city and school can be hard, but it's important to maintain a respectful attitude.
Then, I would encourage you to carefully evaluate what clothing standards you both have. What precisely do you mean by "decent clothes"? Do you mean clothes that aren't stained with sweat and that aren't noticeably stench-ridden to even a person four grocery aisles over? Or do you mean a shirt that doesn't have an obscenity on it? Or do you mean a nicely ironed button-down shirt and khaki slacks? Or something that would pass for a school uniform, such as a clean polo shirt and navy slacks? If he's wearing clothing that is stained with cheese sauce, that reeks, that's one thing. If he's wearing clothes that aren't suitable for church but are clean, in good repair, and don't violate any school standards, maybe you need to drop that standard. His hair might not be nicely styled, but is it clean? Or greasy and matted? When you remind him that it's time for a new sweater, is it simply because you've seen the same one three days in a row, or because the old one is making your eyes water with the smell?
If he's dirty, smelly, and not meeting the basic standards for cleanliness, then you may have to put up with it for a bit. Because the lesson will teach itself. To the immature 13 year old boy brain, not showering or wearing laundered clothes sound like the ultimate "bad boy" rebellion. Yeah, they'll show mom who always made them wear clean clothes. "That's not how we roll, mom" Oh, until a girl winces at the smell of them. Then your next problem is that they buy bottles of Axe body spray and deodorant, and your household monthly budget is shot. And the showers last for 1 1/2 hours. And they spend hours getting their perfectly coiffed hair just right. And they get upset if you accidentally brush the tops of their heads. "MOM! I just got the top right. MOM! Now I have to do it again", and you swear you only lightly touched one molecule of one hair strand, as they storm back to the bathroom.
If he's not meeting school dress code standards, you simply tell him you're not giving him permission to go on any field trips. You withhold privileges at home that would ordinarily be granted to someone who is participating in the household chores and taking good care of their property.
And you make sure to stay available to talk and listen. Don't make every conversation about clothes, showering, hygiene. Speak kindly, and if he doesn't open up, that's ok. Stay approachable regardless. It's so easy to make every conversation about the standards. But make your words about his good qualities, about the weather, about the dog, about the upcoming trip and your first teen field trip, etc. Tell him about your nervousness in 8th grade, or a time when you moved, and how you dealt with it. Don't demand an answer. Just share, and let him know you're human, you're trying to be a good mom, you love him, and you love his _______ (skill with video games, basketball prowess, interest in whatever).