Gryffindor: "All gryffindors are bombastic"- the house of Gryffindor is also the house of chivalry and honour. They're not all loud and unmanageable. There's a difference between standing up when necessary and being a public nuisance
Ravenclaw: "Ravenclaws are very school-smart"- Ravenclaw is also the house of good judgement, sharp wit, and curiosity. None of that ensures good grades; in fact, some of those things have a tendency to make teachers hugely frustrated
Hufflepuff: "Hufflepuffs are wishy-washy"- Hufflepuff is the house of kindness, but also honesty, justice, and hard work. Kindness is not blindness, meekness is not weakness. The choice to be a good person takes strength beyond measure
Slytherin: "All slytherins cheat/play dirty"- there are few things more annoying than having your achievements discredited because people assume you couldn't do it without cheating.