Sure, if the links are relevant. Stack Exchange is a collaborative platform. Editing other people's posts to improve them is encouraged.
Quoting the official guidelines on editing:
When should I edit posts?
Any time you see a post that needs improvement and are inclined to suggest an edit, you are welcome to do so. (…) Common reasons for edits include: (…)
- To add related resources or hyperlinks
If adding a link to Wikipedia improves the post, accept the edit. If the link is wrong or irrelevant, reject the edit. Generally, linking to Wikipedia or some other reference is a good idea in two cases:
- For explanation, when mentioning concepts that are likely to be obvious to some readers, but mysterious to others. (Concepts that most readers are unlikely to understand should be explained in the post, and concepts that most readers would find obvious don't need any explanation.)
- For further information, when mentioning a concept that is peripheral to the post, which doesn't warrant further discussion inside the post but where a curious reader might want to know more.
If the Wikipedia article is relevant but unhelpful or misleading, that's a reason to lead towards rejection. However, the best outcome would be to go and improve the article!
Do not reject a suggested edit or roll back an edit on principle because “it modifies the post”. Edits modify the post by definition. Reviewing suggested edits is a judgement call, that's why we have humans doing it and not computers. But as a human, you're supposed to apply common sense and follow shared values. Stack Exchange has very few firm rules but many guidelines, and for editing in particular there are official guidelines. You should go against the guidelines if a specific case would go against our goal of “build[ing] a library of detailed answers to every question about cryptography and cryptanalysis”. But if you're systematically going against the guidelines, you're doing it wrong.
Different people can have different thresholds for when a suggested edit is acceptable and when it does too much, and that's fine. But if you systematically apply your own rules that go against the guidelines, that's not fine. It disturbs me to see moderators here who completely ignore the official guidelines and make recommendations that go completely against them, given that the job of moderators includes enforcing these rules.