Is "quanto importanti sono i voti" wrong?
It should be correct !
'Come sono importanti i voti?' should be correct as well, shouldn't it?
No because you are asking essentially asking of how much importance are grades, therefore a quantity, therefore 'quanto'.
Why not "quanti"? Sorry i may be misunderstood when to use the plural
That would mean how many grades are important, as in the number of grades, not the importance of grades in general as it is with quanto.