Most of what has been said about the Smiths------(be)also tu

is
这里涉及到两个考点
1.most of + n.谓语动词是单数还是复数根据后面的名词判断,可数名词后面用复数,不可数名词后面用单数
e.g.Most of the students are interested in English.
Most of the rice is for the poor.
2.本句most of后面是个从句,从句一般属于单数.
综合以上,得出is.
意思：大多关于史密斯家的言论也适合约翰家.