I can't reproduce here with either a GTK+ 2 or GTK+ 3 build with "DejaVu Sans Mono Book", although the underscore is well below the other letters, and I've definitely hit this issue before at some point.
Seems to be [fairly common](https://www.google.com/search?q=dejavu%20font%20underscore), maybe it was fixed in `fontconfig` or `fonts-dejavu` or something (`fontconfig` v2.12.6 and `fonts-dejavu` v2.37-1 here).