“Mine as well” is a common misspelling of the expression might as well.
This expression is used alone or as part of a sentence to indicate something that makes little difference either way or that there’s no reason not to do (e.g., “We might as well ask her”). You should write might as well, not “mine as well,” to express this meaning.
On some occasions, mine as well can be the right choice. “Mine” is the first-person possessive pronoun, indicating something belonging to the speaker. So you might use this phrase in an exchange like the following:
Person A: My legs are aching.
Person B: Mine as well. It was a long walk!