Mids may earn ribbons and devices (e.g., jump wings) while at USNA. Most plebes qualify on pistol and/or rifle and wear those ribbons. There may be some ribbons, such as the National Defense Service Medal, that everyone in the military at a certain time was/is entitled to wear. And, as '64 said, prior enlisted are entitled to wear the ribbons and devices they earned while enlisted.
It's funny how military folks can determine a lot about a fellow military person's career with only a quick glance at their collar and the area above their left chest pocket.
From devices, you can determine their warfare specialty (e.g., subs), whether they are in command (gold star), certain special qualifications (jump wings), their shore duty (e.g., CNO staff), etc.
Based on the ribbons, you can tell whether someone has deployed (Sea Service Ribbon) and how many times (little stars for each deployment), whether they were prior enlisted for any length of time (Good Conduct Ribbon), whether they were injured in action (Purple Heart), whether they served in a war theater (various ribbons, depending on the war/campaign), how many staff jobs they've had (which tend to produce different ribbons than "sea" jobs), whether they've done something really brave (Silver Star, Navy Cross), etc. Some only wear the top row of ribbons, which is permissible.
In many cases, you can "guestimate" a person's rank solely from his/her ribbons, as certain ones (Navy Achievement Medal or NAM) are given to junior officers (sea or staff), others (Navy Commendation Medal) to senior lieuteants and O-4s, and different ones (e.g., Legion of Merit) to senior officers. While not universally true, it's generally accurate.
There's a lot more nuance than what I've described above (and there are additional things you can learn about enlisted personnel from their uniforms, such as time served) but, trust me, the average military or former military person can tell quite a bit about a person's career without their ever saying a word.
And, finally, as '64 said, ribbons are worn on most USN officer uniforms -- whites, khakis, SDBs, and SDWs (dress blues and whites). At USNA, ribbons and devices are not worn on "whiteworks" (the sailor uniform) or uniforms worn to class (previously WUB "D" and "A" but now called something else, I believe).
More than you wanted to know, I'm sure.