Question: There have been a lot of edits up for review recently that consist of just adding the syntax highlighting tags to code snippets.

Is the general consensus that this would be considered 'too minor' and therefore rejected, or is it seen as a good edit to make snippets language-specific for syntax highlighting. I am on the fence as to whether to accept these edits or not.

Answer: 
You should check whether the original question is tagged correctly. The tag will provide the hint for the syntax highlighting to work. There are some cases where the tag is there, but the syntax highlighting is incorrect - in which case manual language hint is necessary.

Make sure not to approve any edit that is including the language hint where redundant (the code is highlighted correctly in the first place). You can also reject edits where re-tagging is the better choice. Personally, I only mark the edit as helpful if the syntax highlighting cannot be improved by retagging.

Check whether there is anything else that can be improved, other than syntax highlighting. Usually there are things such as salutations, grammar errors, punctuations, capitalization that can be improved.

As a tip, you can always open the original question and look at the syntax highlighting, and hit the improve button to see how the syntax highlighting changes with the edit to compare.
When reviewing I personally would not reject it if there truly was nothing else to correct. But I don't think it should be the only thing you change if the post has other issues. In that case I would either go for a "too minor" vote, or make the required changes myself, removing the "helpful" checkmark.