To understand if comics offer stereotypes, we have to know the definition. A stereotype is a popular belief about specific social groups or types of individuals, or a perception that is perpetuated by a social group. Every social group has a stereotype attached to it, so they are not exclusive. However, there are exceptions to every stereotype, due to factors like upbringing, aspirations, etc. In terms of comic books, of course they offer stereotypes. Pulp magazines and comics were established in times that were "separate but equal", and segregation was no secret. Therefore, racism was no secret. It would not be until 30 years later that America would even see the notion of equality. Comics offer stereotypes for almost all races because there was no concept of equality, but it does not excuse them.
In terms of race, it's safe to say that all comic book superheroes were whitewashed until the late 1960s. There are hints of stereotyping with white characters as well, such as because of Superman's supposed Jewish influence or Banshee (from the X-men) and his ability to speak Old Irish regardless of being in the 1960s. Not all characters were white, of course, but even in urban populations, it was easy to tell that the demographic was not accurate. It is noted, though, that a lot of the villains were of different races. Egg-Fu from Wonder Woman and Gorilla Grodd from The Flash, for example, were considerably significant hints of racial stereotyping. In comics that did not feature superheroes, most of the enemies were foreign entities. Dell Comics attempted to have an African-American hero, a reboot of the western hero named Lobo. This was a small advancement to the "race" issue. Unfortunately for Lobo, he only lasted for 2 issues and had no super-powers, so it was a smaller step than intended.
The first (or at least, most famous first) ethnic superhero actually came from Africa. The Black Panther was put in a supporting role in Fantastic Four #52 in 1966. Named T'Challa and the heir to a Wakandan throne, Black Panther sported a full body suit in the form of a panther, and possesses enhanced senses with superior combat skills. By his name, it was insinuated that he was black, but by the suit it was impossible for readers to tell at first. It's needless to say that they got the hint pretty quickly. Other characters sported the same "Black" moniker, as if the readers couldn't tell otherwise (like Bill Foster, the "Black Goliath"). Despite that small controversy, Black Panther's reception was mixed. Being from Africa, he could not relate to readers as much as an African-American character would. That would come in the form of Luke Cage, the man with unbreakable skin.
Cage was created in 1972, and gained his powers due to a cell regeneration experiment. Born and raised in Harlem, Luke spent his youth in a gang. This is very relatable, if not a bit stereotypical of black youth. Luke Cage's success over Black Panther was somewhat predictable, and his relevance was only matched by Falcon. Falcon's origins were much more stereotypical than Luke's, however. A native of Harlem, Falcon was a jaded ex-pimp with street smarts and the nickname "Snap". Although becoming a superhero in the end, Falcon's past is about as stereotypical as it gets.
In the end, some ethnic characters were created with good intentions, and others were used as propaganda. Storm was recruited to the X-men about 3 years after Luke Cage, and is now a core member of the team. Recently, the introduction of Miles Morales as the new Spiderman has caused some controversy. Readers worry that Miles' ethnicity will have nothing to do with his significance, but the fact that he is Latino/African-American alone has major importance. With my limited knowledge of character profiles/ethnicities, it's hard to say just how many minority characters are out there. I've only seen…maybe 30 characters of different ethnicities, villains not included. But it is evident that the comic book world is trying.