I am removing outdoor carpet that was applied over 5/8 plywood with Mastic. There is some residue of the mastic on the plywood. Now I want to put down Ditra mat in thinset and then tile on top of that. Can I do that or do I need to resurface the sub floor?

I recommend you add an underlayment grade plywood of min. 3/8", preferably 1/2". 5/8" plywood subfloor is bare bones minimum for tile, and then ONLY if its deflection is not more than L360. Who knows what your numbers are, plus not a good idea to take a chance with that water base or latex base adhesive residue you've got there. Smart move with Ditra though.

Jaz