Does tier 1 or tier 2 depend on hardness of problem?
Is it true that publishing anything in theory in tier 2 ( let's say ) is equivalent to publishing something in tier 1 in application side?


Answer: No. It is not true.

I assume that you mean to say "Theory" means "Theoretical Computer Science" that includes areas such as Automata Theory, Algoriths, Computational Complexity, Learning Theory. And, "application side" meaning to "some areas that uses concepts" such as Machine Learning, Computer Vision etc.

Such a comparison is not possible. And, as far as I know, there is no venue or organization that compare two people from two different research fields compare on the aspect of their publishing venue. That would be highly ambiguous.

These conferences are meant for their own fields. For example, application side of Computer Science such as Machine Learning or Neural Processing have their Tier-1 conferences such as ICML, NIPS etc. For Theoretical Computer Science, we have ICALP, SODA, MFCS.