This is a very good idea and one that is support be implement, we actually do not loose karma by giving karma, either it be positive karma or negative karma, but it can still as well be painful, or regretful when one mistakenly gives a users either a positive or negative karma, that is positive karma to an undeserving post, or negative karma to also an undeserving post.
And also think of the fact that the both karma buttons are so close to each other, and for anyone on mobile, there is also the possibility of mistakenly clicking a negative Karma when you intended clicking the positive karma button, and also clicking the positive when one intended clicking the negative Karma button.
All of such possibility of mistakes can be easily prevented by introducing a pop up dialog box where a user will be asked to confirm their actions before it's actually carried out by the system, same way its done on btt when ever one is to give merit to another user.
This is a minor feature but one that could really help anyone at some point In the future.