I jump to opening paren and type #_ , or slurp into a (comment ...) form. In general, I avoid using semicolons to comment out code, because it's easy to lose in-between code / paren balancing that way.

Try C-x C-;

I’ve been working on this mode https://github.com/dotemacs/clojure-comment-dwim.el/blob/master/clojure-comment-dwim.el you might find it useful.

I'd forgotten about (comment) . C-x C-; doesn't work because that comments a single line and not the whole form. Thanks @U3SG7RX7A too!

I move at the beginning of the form, C-SPC, C-M-f and then M-;