The error message is not indicating that the public key is missing but rather that the signature is bad.
I too have same warning message, I have tried all solutions provided here, but nothing works.
The possible workaround is Install from tarball:
I am just copy & pasting commands here from above link.
Download the binary files for the desired release of MongoDB.
curl -O https://fastdl.mongodb.org/linux/mongodb-linux-x86_64-3.2.0.tgz
Extract the files from the downloaded archive.
tar -zxvf mongodb-linux-x86_64-3.2.0.tgz
Copy the extracted archive to the target directory.
mkdir -p mongodb
cp -R -n mongodb-linux-x86_64-3.2.0/ mongodb
Note:
Don't just add keys from error messages. Only add keys you trust. Because any repository with this key, would be able to install any package without warning.
EDIT:
mongodb 3.0 has no warning messages,
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
echo "deb http://repo.mongodb.org/apt/ubuntu trusty/mongodb-org/3.0 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.0.list
sudo apt-get update