it's not really funny to be like that.
=【It 形式主语】【is 谓语】【not really funny 表语】【to be like that 真主语】.
=To be like that is not really funny .
要用to be like that是因为,它是【不定式短语】,在这个句子中做真正的主语,而like that是介词短语,不能作主语.
祝你开心如意!