Can you describe a simile as an ambiguous comparison or not, I don't know whether it quite makes sense?

What an interesting question. Certainly makes a person think and this person thinks that most similes are ambiguous, though I wouldn’t call them comparisons at all. Similes in my mind are various interpretations of a word. Similes can’t be ‘certain’ as languages have such different usages of words, i.e., we can’t get stuck in the English language;-)

You can, yes, but it depends on the simile.

“I’m like an ox” by itself is an ambiguous comparison (and, obviously, a simile).
“I’m dumb as an ox” (or the less grammatical “I’m dumb like an ox”) is much clearer / unambiguous.

And, grammatically at least, similes are comparisons.

So do you think i would be able to get away with saying this in an essay?:

Although to use a simile is just to say something is like another thing, similes can sometimes be used in descriptive writing to make an often slightly ambiguous, somewhat unusual comparison between two rather different things.

It’s not as accurate as it could be but do you think it makes sense?

I think it does. When you consider the meaning of ambiguous (“open to or having several possible meanings or interpretations; equivocal”), I think you can argue that similes—and metaphors, perhaps even more than similes—can be used in such a way as to introduce ambiguity and use it to some literary purpose. That might not be the most usual use of simile, but I would certainly say it is a possibility.

To say that it makes sense is not necessarily to say that it is correct (or that I agree with it) but only that it is a sensical statement that can be logically defended. I presume that your essay then goes on to do just that, using an example that supports your case.

By the way, you don’t need both “sometimes” and “often” as qualifiers. That’s redundant. “Often” is an adverb of time, and you are not really talking about temporal frequency here but of commonness. “Sometimes” takes care of that and is perhaps a better choice because this use is not actually so common, as you rightly point out with “unusual.” Better delete “often.”