Having run my MBP 2017 in clamshell mode, I've noticed that the internal temps are way higher than when I have the lid up. The keyboard acts as a heatsink, in a way, imo. I probably wouldn't put a silicon cover on the keyboard primarily because the keyboard is so thin, the silicon has to only make it worse, that and the heat issue.
I used one on my 2015 MBA and it made the typing so bad I had to get rid of it. That and it put imprints on the screen.