OH suggests that she might have been what in my youth was called "simple"?
This, I think. From the OED:
innocent, adj. and n.adj. 3 b. Deficient in intelligence or sense; silly, half-witted, imbecile: cf. B. 3b. Now dialect.
1548 Hall's Vnion: Henry VI f. clxix That he was either a childe, whiche had nede of norice..or an innocent creature, whiche muste be ruled by a tutor.
1548 Hall's Vnion: Edward IV f. ccx He was a man of no great wit, suche as men comonly call an Innocent man, neither a foole, neither very wyse.
1688 in J. Barmby Churchwardens' Accts. Pittington (1888) 342 To John Dods for keeping the innocent boy, 1l.
1706 Phillips's New World of Words (new ed.) Innocent, inoffensive,..harmless, also simple, or silly.
a1825 R. Forby Vocab. E. Anglia (1830) (at cited word) ‘An innocent man’..is an extremely common expression for a silly fellow.n. 3. (a) A guileless, simple, or unsuspecting person; one devoid of cunning or artifice. (b) One wanting in ordinary knowledge or intelligence; a simpleton, a silly fellow; a half-wit, an idiot.
c1386 G. Chaucer Canon's Yeoman's Prol. & Tale 523 O sely preest, o sely Innocent With coueitise anon thou shalt be blent.
1578 J. Lyly Euphues f. 34 In bodye deformed, in minde foolishe, an innocent borne.
1593 R. Harvey Philadelphus 91 That he might do what he list in the kingdome under such an innocent and milksop.
1598 Faversham Par. Reg. (MS.) Buryed, Margery, an innocent from the Abby.
1694 R. L'Estrange Fables (ed. 6) ccccxxxviii. 475 There was just such another Innocent as this, in my Father's Family.
1706 Phillips's New World of Words (new ed.) An Innocent, an Idiot, or Ninny, a silly, half-witted Person.
1814 W. Scott Waverley I. ix. 123 ‘He is an innocent, sir’, said the butler... Waverley learned..from this colloquy; that in Scotland..a natural fool [was called] an innocent.
1838 G. P. R. James Robber I. vi. 118 The man is a poor innocent whom I have known this many a year.