Why is fickler a word?

Solution 1:

Practical English Usage by Michael Swan (Oxford, 2005) has

two-syllable adjectives

Adjectives ending in -y have -ier and -iest.

happy happier happiest
easy easier easiest

Some other two-syllable adjectives can have -er and -est, especially adjectives ending in an unstressed vowel, /l/ or /ə(r)/.

narrow narrower narrowest
simple simpler simplest
clever cleverer cleverest
quiet quieter quietest

With many two-syllable adjectives (e.g. polite, common), -er/-est and more/most are both possible. With others (including adjectives ending in -ing, -ed, -ful and -less), only more/most is possible. In general, the structure with more/most is becoming more common. To find out the normal comparative and superlative for a particular two-syllable adjective, check in a good dictionary.

Fickle is pronounced with an unstressed final /əl/, so Swan would allow it to qualify for fickler, although as he says, "the structure with more/most is becoming more common."