If you mean wrapping the whole thing in foil, absolutely not. Depending on the age of the router, for example a router from the late 80's which has an internal fan for cooling, you could damage it beyond repair and it could overheat.
Some people think it helps to hang a small foil "flag" from the external antenna.
In theory, it would help if you cut the foil exactly to the exact size. But the likelihood of doing more harm than good may be very high. This goes back to the days of broadcast television before cable TV. Every baby boomer will remember these, along with parents instructing their ORC devices (aka kids) on how to turn/position the antenna for best reception, as well as getting up and changing channels.
But broadcast television operates on the VHF band between 54-216 MHz, which is the only channel where foil flags are useful (although, as an adult, I wonder how helpful they are, rather than in the vain pursuit of picture perfection.
At WiFi frequencies, which are 100 times higher than the highest VHF channel, so the wavelength is 1/100, about 125 mm, you can never make a sign precise enough to be effective.
