I agree with you that the priority should be on mechanics. And most indie jam participants aren't impressed by lights and effects as they are developers themselves and can tell if you didn't put much effort into it.
But I use game jams as like a mini test for what I'd like to do as I establish and improve my workflow, using the simulated limits that the jams provide. (Timeline, theme etc.)
That's why all my games, no matter how small have full menus, volume sliders, pause functions etc. I know most game jammers don't care and won't use them. But I personally cannot release a game without those aspects, even if its just for a game jam.