I think there are two issues with your construal of the semantics of moral language.

First, the literal translation from "x is wrong" to "I don't like x" needs to be supplemented in a number of ways, and a lot of these supplementations are not immediately clear. For instance, people will say things like "Last week, I didn't like eating meat. I recently changed my mind" without intending to say anything like "Last week, it was morally wrong to eat meat. Now it isn't wrong." So you need some explanation of how to handle adverbs of this sort. Of course, this is just one example. There will be other adverbs that cause trouble for you. You could try to produce ad hoc explanations for any problem case, but this isn't actually how semantics works: it's compositional. We need some general story about how the adverbs in sentences including moral terms actually impact the meaning of the deeper sentence (about one's own preferences) that you think is actually being expressed.

Okay, that's one problem. Here's another one, based on my foggy recollection of an essay by Mark Schroeder. The gloss "I like (approve of, whatever) X" has two slots for negation. You can say "I don't like X," "I like not-X," and "I don't like not-X." But our moral language has more slots for negation than this. For instance, I can say "X is permissible" "X is right" "X is wrong" or "Not-X is permissible" "Not-X is right" "Not-X is wrong."

Okay, so let's try to give our expressivist gloss for each utterance. X is right -- I like X X is wrong -- I don't like X Not X is right -- I like not X Not X is wrong -- I don't like not X X is permissible -- ?? Not X is permissible -- ??

We can't just use negation in a natural way to offer an expressivist gloss here. We've used up all our negation slots, or whatever. The gloss "I like X" does not have enough structure to differentiate thinking an act is morally permissible from thinking an act is morally right/wrong. That's bad. We want our semantics, again, to have pretty clear compositional rules that can properly account for negation in the surface grammar. Your semantics just doesn't have enough strength to do that.

Ok, so the natural move is to introduce another attitude -- say tolerance. The statement "X is permissible" actually just means "I tolerate X." This is a route you could take, but the problem is that you're just doing more damage to your semantics. We want our semantics to explain why "X is wrong" and "X is not wrong" (or "X is permissible") are incompatible. Your semantics, which would now offer the glosses "I don't like X" and "I tolerate X," requires an explanation of why not liking and tolerance are logically incompatible attitudes (that is, why no one can ever not like something but still tolerate it).

Maybe this is possible, but it's certainly hard to do. Since your kind of expressivist needs to invoke a distinct kind of attitude in order to explain the negation of moral sentences (not all brands of expressivism have this problem, though), you take on the significant burden of explaining how these apparently quite distinct mental attitudes can be logically incompatible. You lose the very easy explanation available to the cognitivist, which is just that there's only one mental attitude here (viz. belief).