Mathematica does not support any kind of comment-to-line-end specaial character(s). You will have to resign yourself to living with that. It does support a very useful and easy to apply block un/commenting feature.
I am running on OS X, so the keyboard shortcut for the context (mouse-right-click) menu's Un/Comment command is Cmnd+/. I use that do any kind of line or block un/commenting. Here is how it works.
Suppose I have a line that looks this this
When I click anywhere in the line sufficiently many times (three times will do it if I click on
data) and type Cmnd+/, it changes to
When I triple-click on the left-most parenthesis three-times, it reverts back to