I think there are different questions here that are getting mixed up.
Are there clever nurses?
Yes! Absolutely tons of very smart people becomes nurses and use their intelligence to provide incredible care.
Do you have to be clever to be a nurse?
No, not necessarily in the conventional academic sense.
I know of several people who have gone into nursing who have worked very, very hard to qualify - but having either worked closely alongside them previously, or gone to school/college with them, I can say categorically that they're not the most gifted intellectually. That doesn't mean they aren't great nurses.
But you could say that about lots of fields. My brother has a PhD in Physics which makes him sound like a huge brain box. He isn't. He barely scraped through GCSEs, didn't do A-levels, failed his professional qualifications when he joined a telecoms firm and then when he returned to education later, he had to work extremely hard to scrape a 2:2 degree. The structure, time and set-up of a PhD really suited him and although - again! - he only just squeaked through, he got his doctorate.
What my brother can do extremely well is research thoroughly, analyse a situation, sift through facts, and consider all elements before reaching a decision. I really value his opinion and consider him to be intelligent, even though he's not conventionally "clever".
There are lots of career paths in nursing nowadays. But equally, there's nothing wrong with not progressing. Doctors and nurses are different careers and require different skillsets, quite aside from any academic "cleverness". Choose the path that appeals to you. The cleverness - or lack of it in some cases - in your nursing colleagues isn't relevant.