Typically, people pronouncing betta incorrectly are not going so because of accent or dialect. It is because they simply do not know the proper pronunciation. Regardless of accent, every word has a pronunciation; you can find this in just about every dictionary out there. I don't think anyone would mind if someone pronounced "betta" as is appropriate for thier dialect, but when someone pronounces it utterly wrong even for thier dialect because they are mixing it up with another word - beta - it suggests that they are at least uninformed. I don't think anyone has a "prejudice" against them, but we must face it; pronouncing any word incorrectly makes a person come off as unintelligent. What do you think of people, for example, that say "libary" instead of "library?"