Oh man, I've been in your shoes with the same worry in the past year, and I went diving into the published studies to see what I could find.
There are several studies which say that a lining over 14mm is not ideal, but there are other studies that say it doesn't affect pregnancy rates. So it doesn't seem to be clear cut either way.
In the end I chose not to stress over it, because even if it really wasn't ideal, it is not like nobody ever gets pregnant with a non-ideal lining. Although we don't see too many thick linings around here, I have read about many on these boards with thinner than ideal linings who had success. And embryos can even implant without a lining at all, e.g. ectopics. Honestly, I think the biggest factor is still the quality of the embryo.