AFAIK the width of fender shall be 10-12mm wider than the tyre. But would it cause a problem if the fender is even wider? Like having a 32mm tyre with 52mm fender?
That's not a problem if you are able to fit your fender onto your frame. The only downside that I can see is that it will slightly increase your air resistance. However, never forget that the biggest source of drag is always on the saddle, so the effect of a wider than necessary fender is rather negligible.