While you and your editor are busy getting the text right, the salespeople are starting to think about selling the book, and to that end they will prepare a jacket (for a hardback) or cover (for a paperback) many months in advance of publication. Your contract will probably say that you will be consulted about jacket/cover design, but consulting means just that: the publisher has the final say and publishers vary enormously in their attitude to author input.
If they produce something you don’t like or weren’t expecting, try to be objective about it. You should certainly point out if they have got the concept of the book wrong e.g. if it is aimed at teenagers and the cover is more likely to appeal to seven-year-olds, or if you have written a work of literary fiction and they have made it look like a thriller. But otherwise, imagine that you are a book buyer for a large chain store, seeing a million covers a day and spending perhaps ten seconds considering each one. Would this cover attract you and make you want to stock the book in your shops?
Put a colour print-out of the cover on a shelf or mantelpiece, with a couple of other books alongside it, and look at it from the other side of the room.