I think that's some very nice figure painting, indeed, especially the face. Getting that much animation from a DML figure head is quite difficult, and, in my opinion, you have done an excellent job.
In regards to the fit of the joint at the arms. I'm guessing that you painted the arms separately and glued them on later. I do this myself very often so that I can paint around the details on the torso, etc.
The key to doing this is dry fitting the arms before painting so that there's as little seam as possible to deal with later.
Sometimes I don't get it just right and have to do some filling and sanding after the arm is glued on. When this happens, you just have to "bite the bullet," fill and sand, and do some re-painting.
However, once the arm is painted and glued on, if the dry-fitting was done well, then you only have a very small seam left to deal with. I think this is the case with your figure. From what I can see in the photo, the arm seams are pretty fine.
What you can do now is use some thinned PVA (white glue) to fill the seams. The PVA dries transparent and flat (matt), but it usually takes several coats to fill up the seam nicely (because of shrinkage).
If you get any excess thinned PVA on the figure, this can be wiped away with a damp tissue or cotton swab (ear bud) after it dries. You can also use thick PVA and fill the joint and wipe it smooth before it dries, but I find this a little messy and less controlled than flowing thinned PVA into the joint.
Once the seam is filled and dry, because the PVA dries clear, it will "take" the surrounding colors. Apply a coat of clear flat and the seam will disappear.
This is a trick used by model airplane builders for canopies and it also works well on clear parts for armor models, like headlight and tail light lenses.
My complements to you on your painting, though. It has a very nicely animated look with precise details.