It depends on what the employer has asked for as regards completeness of information on their application form.
YABU....if they have asked for full details of education, training and experience including full career history and explanantion of any gaps. By not answering fully or omitting details specifically asked for i.e. complying with their terms - you are being dishonest
YANBU (possibly)....if you are only asked for career details and certificates or qualifications relevant to that particular job.
If I was looking to employ someone I would expect honesty from the start.
Also difficult to comment fully without knowing what the job is, what your qualifications are and why you consider yourself "over qualified".
As an aside, it may be that some of you who regard themselves as "over qualified" are misunderstanding the requirements of a particular job. For example it's all very well having a degree in English if, for example, the job calls for NVQ Level 1 plumbing. Your qualifications may be of a different level but in fact you are underqualified for that particular job.