The int stat represents a combination of both education and actual intelligence, the average Imperial citizen is unlikely to have much more than an elementary education and some vocational skills hence the 20-25 average "intelligence". The RT, Navigator and Seneschal are all likely to have high Int because their backgrounds tend to include really high end classical educations. The Astropath and the Explorator are both products of grueling mental training and in the latter case possibly an augmented brain. The Voidmaster and the Arch Militant aren't dumbasses unless you want to roleplay them that way they're just much more blue collar and physical.
The missionary is probably pretty dumb though, likely having never read a book that wasn't about burning aliens and mutants and possessing that practiced stubborn stupidity that is such a defining trait of screaming fundies.
Having an Int of 15 doesn't mean you've gone full retard, you might very well be kind of Forest Gump retarded but it's just as likely that you have a third grade education or some combination of the two. Unless you're an Ogryn if you're an Ogryn then yeah you should roleplay like you're a retard.