There aren't that many Americans in-universe. Of the original crew, only Kirk, McCoy and maybe Sulu? Chapel?
On TNG, Riker is from Alaska. Geordi may or may not be from America. He was probably raised on starbases. Picard, Crusher, little Crusher, Troi, Data, O'Brien, Ro, etc are not Americans. Maybe Barclay?
On DS9, only Sisko is an American.
On Voyager, Janeway is American(because she gets her name and origin from Kate Mulgrew) Paris may or may not be American. He was probably raised on various Starbases and attended the Academy in France. Kim may or may not be American. The rest are definitely not American.
Enterprise: Archer was from New York, and Tucker from Florida. The rest were not American.
Certainly American: 7(Kirk, McCoy, Riker, Sisko, Jake Sisko, Janeway, Archer, Trip)
Possibly American: 6(Chapel, Sulu, Laforge, Pulaski, Paris, Kim)7 if you include Barclay.
Possibly not: 6(or 7)
Certainly not American: 32(Spock, Uhura, Scott, Chekov, Picard, Dr. Crusher, Acting Ensign Crusher, Worf, Troi, Data, O'Brien, Guinan, Ro, Kira, Jadzia, Bashir, Odo, Quark, Nog, Rom, Ezri, Chakotay, Torres, EMH, Neelix, Kes, Seven of Nine, T'Pol, Reed, Hoshi, Mayweather, and Phlox.
Are Americans over represented on Star Trek? No, not really.