" when it comes to love even the smartest people become stupid"
The often heard "Liebe macht blind" is close but not a perfect fit. Blind in its literal sense is just one form of sensory input, not really the whole of cognitive processing being impaired. Further: this blind is usually directed towards the 'object' of love and desire, not so much towards everything in general. Although both meanings might be understood in more generalised terms, the reverse translation of "Liebe macht blind" would be "Love is blind".
For the actual question:
You might use a direct translation, like:
Wenn es um die Liebe geht, werden selbst die klügsten Menschen dumm.
This is beginning with what you are looking for, if we only take the question's headline. But that sounds even less like a proverb than the English original.
To get into idiomatic territory and addressing more the body text of the question, this has to be shortened to something like
Liebe macht die klügsten dumm.
Or even shorter