No it does not mean you are dumb! A grade 5 is a strong pass (If you translate to the old grades that a high C which is good)
If you personally want to achieve higher look back at your test and look at which topics need improvement on and study them more, a great learning source is Mr Barton maths who give you links to a question sheet and another link to the answers. One other great source is Hegarty maths with different videos on the topics and little tests to complete. If your school does not have hegarty maths I highly recommend you ask because it will help a lot.
Do not worry about receiving a grade 5 though because its a fantastic pass!