Yes, USB signals are less noisy.
USB's main claim to fame is that it is differential. Instead of having a signal referenced to ground (this is called "single ended"), there are two signals referenced to each other. When one is high, the other is low. This keeps the circuit loop very small, which makes it harder to couple inductively. Since the lines are close to each other, anything that capacitively couples will couple to both lines; so any noise injected onto one line is also injected onto the other line, and the differential receiver can reject it (the receiver's ability to do so is referred to as "common mode rejection range")
Imagine you have two wires, one is 5V the other is 0V. Now lets say a noise spike injects one volt onto both lines. Now they're 6V and 1V. A single-ended receiver would see 6V as a result of this noise. A differential receiver sees the difference between 6 and 1 is the same as 5 and 0, therefore it doesn't see any noise.
What does it look like on a scope? While Russell's eye diagram looks pretty, the eye diagram is more of a diagnostic tool, and not an example of real-world data transmission.
A differential "1" is D+ high and D- low. A differential "0" is the reverse, D+ is low and D- is high. So if you hooked it up to a scope, you'd see the two traces move opposite of each other; when D+ goes high, D- will start going low simultaneously, and vice versa.
Beyond this point, it gets terribly complicated. For instance, USB is an NRZI protocol. Instead of a differential 1 representing a "1" bit from the PC, a "1" bit from the PC is encoded as a transition. So a 1 bit would be either a differential 1 transition to a differential 0, or a diff0 transition to diff1. Meanwhile, a "0" bit is encoded as no transition. These transitions are important because USB's clock is implicit in the data, therefore the USB protocol requires bit stuffing (the act of inserting "fake" "1"s into the data stream) to ensure that a transition happens often enough that the clocks can stay synchronized to each other. Without bit stuffing, a very long string of 0s could result in a loss of synchronization; the host could send 500 "0"s and the device receives 499 or 501 "0"s.
RS232 also has an implicit clock in the data. However, instead of bit stuffing, the start and stop bits will ensure that a transition occurs every so often.
On top of bit stuffing, you have the rest of the software protocol to worry about. RS232 is pretty much just an electrical standard that determines how you signal 1 and 0, and it leaves interpreting those 1s and 0s up to the devices. There aren't any headers, for instance. USB is quite different. A small portion of the USB traffic is not your data, but instead "management". For instance, a data packet has a 16-bit CRC of the data you're sending; this CRC will be used to re-try any non-isochronous transfers that have errors. There are even other packets that are totally unrelated to your device going around on the wire, for instance the sync packet that goes out at the start of every microframe.
Even the very act of sending data to the PC requires first that the PC send an IN token to the device. The PC owns the wire, and the device is not permitted to send any data until the PC asks it to. This IN token is another example of additional traffic on the data line that USB requires and RS232 does not.
