Tiling a floor

Yes, but inadvisable to do so directly unless its absolutely stable and free from any movement, and even, to boot.

Thats why most people screw down a ply sheet over it. If that makes the floor unacceptably high, then you should remove the boards and re-floor with the ply. This gives you the chance to pack out the joists with slivers of e.g. thin ply (use a spirit level) to get them totally flat and even.

Then use a fairly thick bed of high quality flexible tile cement, and wait a week or three before grouting if you can, to allow any dampness and swelling that the cement make have introduced, to subside