It seems a very American thing for cities to have mottos (mottoes if you prefer) - and many US cities even have more than one. San Diego, for example, confidently claims to be 'The Place Where California Began', 'Plymouth of the West', 'The First Great City of the 21st Century' and, perhaps most contentiously, 'America's Finest City'.
British cities also have mottos, but they tend to be in Latin, or unknown to the general public, or both. Wolverhampton's motto is 'Out of darkness cometh light'; Sheffield goes the Latin route with 'Deo Adjuvante Labor Proficit' (with God's help our labour is successful); Birmingham laconically chooses 'Forward'.
My nomination for the British city with the worst motto is Derby (which, for all our non-British readers, rhymes with 'Barbie'). Its motto, in full, is 'DerbYes! The city where you can'. My first objection is to the 'DerbYes' part. I am assuming this is a combination of 'Derby' and 'yes', but it is easy to read it as the nonsensical two-word phrase Derb Yes, or possibly as a misspelling of 'Derbies'.
Secondly, 'the city where you can'? Can what? Get mugged? Stabbed? Be homeless? Jobless? Even assuming the motto writers intended something positive, what is it that you can do in Derby than you can't do anywhere else? Except, perhaps, be ashamed of having the worst motto in the country...