That I love you is a surprising thing有错吗

没有错

这是that引导的主语从句

that后的从句整个作为本句的主语

类似的主语从句还有：
Who will be our teacher hasn't been decided yet.
Whatever you did is right.

如果说成The fact that i love you is a surprising thing 就没错了

肯定没错啦
这是主语从句
THAT + 从句 充当主语

是正确的 是一个主语从句
I love you 是主语

"Is it a surprising thing that I love you ?"
that 引导的从句做主语，用it 代替，避免头重脚轻。