Can you apply to an Ivy League college as an undecided? Does it impact the chances of being admitted?

Yes, of course you can. The official stance is that it does not impact your admissions decision and I am inclined to agree with that from personal experience, though there are few caveats.

Firstly, if you're applying to places like Cornell and Penn that have specialized schools for engineering, architecture, and the arts and sciences etc. then I guess that it does impact your decision in the sense that if you apply to the engineering school at Penn and get in, then it is hard to (not impossible) transfer to Wharton and do business whereas at places like Harvard, Brown, and Yale you are just admitted to the college and can then major in anything you want.

Secondly, lots of colleges often have questions linked to what your interests are and how you may pursue them at their university. It is possible to answer these questions while also conveying the message that you are undecided and would like to explore different fields of study (that is what studying in the US is all about) but it may sometimes be difficult because it is much easier to answer such questions if you can show that you have some clear direction/ambition/passion.

However, excluding these reasons, I don't think it will impact your admissions decision in any way at all.