Are multiparty systems necessarily more representative than the two party system in the United States? why or why not?

For the most part, I would say that they are more representative, but I would not say that they are necessarily so.

They are generally more representative because in those systems, smaller parties can more accurately represent the wishes of their members.  For example, if we had a "Tea Party Party" in the US, it could accurately represent its members' attitudes without having to compromise.

However, in such systems, parties might have a harder time representing people who feel that there are many important issues.  For example, if we had an anti-abortion party and an anti-tax party and an anti-environmentalism party, what representation would there be for people who think all of those are equally important?  Or for those who think that two of the three are but one is not?

Voters all have different mixes of issue attitudes and issue salience and even multiparty systems cannot accurately represent everyone.

One other issue to consider is that, in a multi-party system, you are often voting for slates of candidates rather than being able to specifically vote for a given person.  You could say this is less representative as well.