A quick glance at the questions using these tags suggests that instead of synonyms, perhaps the first step in straightening this out is just re-tagging to use the accurate mechanical terms for the system each question is asking about. I don't yet see any question about the generic concept of a fumble or natural 1 fail; just questions about specific mechanics in specific systems. So how about we tag for the games' own terms?
Tagging for specific mechanics should also increase the likelihood people Googling for help with a particular game will find our site! That's part of what tags are for.
Look at the suggested synonyms here: few if any are mechanical terms, and for good reason. Synonyms are "alternate spellings or phrasings," not replacing one mechanical term for another. I don't think we'd want to force Fate questions to use the feats tag instead of stunts, for example.
So when we need a generic tag for the concept (and we don't yet), lets find a way to do it which lets questions about specific game systems also use those systems' designated terms.
The concept of 'An abysmal failure that leaves you worse off than if you had not attempted the thing in the first place' (critical failure), and 'things have gone horribly wrong' (fumble) are distinct concepts in some games.
FFG's Star Wars (for example) allows actions that result in Success, Success and a Fumble, Fumbles, and Critical Failure without a Fumble (translating similar concepts to the tags in question).
I would be hesitant to jump on merging the two.