Yes it is. The
DOI is an identifier similar to an
URL that will likely be transferred from the printed document by typing it into some search field.
As in a
URL it is important to have clearly distinguishable letters as one can in many cases not distinguish between some characters from the context (compare for example the minor "L" and the capital "i" that look the same in many fonts). Typing the wrong character will in most cases lead to a wrong or not existing document. Therefore the
DOI is, similar to a
URL, printed in a typewriter like font, where the letters are clearly distinguishable.