I think using 又小又可爱 is more idiomatically Chinese.
I can also find that 她的嘴巴很小，也很可爱。 is also correct in terms of emphasis.
As a parent of 1/2 Chinese children, this is something I heard adult women, aunties, mothers, and grandmothers would often say when complimenting a baby or small toddler.
"YOU GOT A PRETTY MOUTH!!!"
他的嘴巴又小又可爱 why not?
You used His 他，rather than hers 她。
I guess the 很…很 repetition is by analogy with 又…又.
I assumed that the second one was superfluous.
I wrote Ta de zui ba hen xiao he hen ke ai. Why is this a problem. It is correct. Why is it wrong to put the AND in the sentence, as you might in English - and as I thought you could in Mandarin. Is there a rule here I should know about????
和 binds nouns, not predicates.