Grammar purists will insist that the comparative form of fun is more fun. That’s because for centuries, fun was used as a noun (or a verb), and nouns never take an -er ending.
The use of fun as an adjective appeared in the early 20th century, and funner, according to Google, didn’t appear until the mid-20th century, but even then it wasn’t until the 1980s that it started to catch on.
That being said, Google also reports that more fun is preferred by a long shot:
Even though funner is gaining traction, it has a long battle ahead if it ever is to become the standard.
My personal opinion is that since funner is a word with fairly widespread usage, indicated not only by the graph above but also by its inclusion in several dictionaries, it should be considered acceptable.
Plus it fits my easy, 3-part rule for remembering comparative and superlative forms:
- For 1-syllable words, just add -er/-est.
- For words of 3 or more syllables, use more/most.
- For 2-syllable words, it’s usually safe to use either method.
The last one is actually a bit more complex than that, but it’ll get you by.
I personally use it (and it’s superlative cousin, funnest), but mainly to prove a point. If you do choose to use it, keep in mind that many professors, editors, and other language experts will consider it incorrect. Tread carefully.
In a more informal setting, have fun correcting your friends when they try to correct you for using words like funner.