The Oxford English Dictionary (OED) indicates that fiddle the verb (first OED citation from 1377) is derived from fiddle the noun (first OED citation "c1275", second from 1377).
If that is correct, then the -le cannot have been added in English as the frequentative verb suffix.
Unfortunately, the etymology of the noun seems to be unclear. The OED gestures at Latin "vitula, vidula", which looks to me like it has the common Latin diminutive suffix -ula. There was a question on Latin SE about vitula.