I haven’t cut that design, so can’t tell you what the backside normally looks like after it’s done, but for designs like that one in general, the kind that have a lot of closely spaced cuts and intricate details…it’s very possible to get some overburn in places.
To avoid it, there are a couple of things you can do…the burning on the backside is caused by flashback, which is fed by air… you can reduce it by sticking sheets of masking tape down on the bed itself, and making sure that the material is perfectly flat on the tray. (I’m not talking about it “looking” flat…you have to make sure. Even 1 mm of warp is going to affect the results and increase flashback.) There are some free pins designed by @eljefe4 that you can cut here to force any warp out of the material…they help a lot.
So your original question was whether that was the best possible result you would be able to get using Proofgrade, and a Glowfirge file, and pre-set settings. The problem with flashback can occur on any file, and it has to do with the nearly imperceptible warp that can appear in wood of any kind, unless you know how to deal with it. We do that by keeping the material perfectly flat on the bed to cut down on the air feed underneath.
The next time you cut it, you might not see the same results. So no, it can look a lot better than that. Make sure the material is pinned down completely flat, all around, and for detailed cuts, it can help to block the air in the cells of the tray. 