A Google Books search yields two matches for mismodify. The earlier of these is from Mervyn Falk, A Cleft Palate Team Addresses the Speech Clinician (1971) [combined snippets]:
Keep in mind that we have defined the production of a speech signal as a modified voiced or voiceless airstream. Now what is done to overmodify or mismodify, as you might conceive of it, has really only to do with getting that child into the therapy situation. From here on, we are interested in what is correct.
And the later is from Rivista Di Linguistica, volume 12 (2000):
3. Adverbs and movement in Romance
Morphologically motivated head movement creates situations of surface mismodification—situations in which an adverb is not next to, and hence not adjoined to, its modifiee in surface structure. So in the now standard Emonds/Pollockian (Emonds 1978; Pollock 1989) surface structure of French we find (8a) analyzed as (8b), where the adverb appears to mismodify the VP, as it does not seem to be adjoined to it in the following: ...
This second example, as you can see, also contains an instance of mismodification, as do eleven other Google Books matches going back to this one in Modern Exposition: A Textbook in Expository Writing (1942):
A particularly crude type of mismodification appears in
The persons the engineer is addressing, either oral or written. . . .
Since oral and written are adjectives and can modify only substantives, they must modify engineer, or possibly persons.
Joel Bowman & Berndine Branchaw, How to Write Proposals that Produce (1992) identifies "mismodification" as a type of grammar error:
Misplaced modifiers modify the wrong word, phrase, or clause. A dangling modifier has no word, phrase, or clause that it can logically modify. Avoid mismodification by placing the modifier close to the word, phrase, or clause that it modifies.
Google Books searches also turn up nine probable matches for mismodified from as early as David Ebert, Synthesis and Localization of Lysosomal Enzymes in Secretory Mutants ... (1986), which uses the term on five different pages, including page 49 [combined snippets]:
Other proteins, essential for the operation of the lysosomal system, may function improperly because they are mismodified, or something may be wrong with the targeting and secretion pathway which affects lysosomal enzyme modification, but is independent of it.
So a whole family of words built on mismodify has existed for several decades, serving writers functioning in fields ranging from speech therapy to biochemistry to oncology to grammar. Most of the instances that Google Books searches identify, however, are clustered in the last two areas.