I am developing a MEAN app a windows laptop. Should I use Ubuntu or windows server on my cloud provider? Are there downsides of choosing one over the other?
I dont see downsides of using your Windows laptop. Its a matter of preference. If you know how to deal with Windows Environment variables, paths, EOL characters, you should be good.
Many developers use Mac or Linux because they deal better with *unix like systems or because similar to the server they will run their apps.
At the end of the day after you build your MEAN solution the output should be the same regardless the OS.
Windows Server or Ubuntu server are just a OS to 'professionally' run your app. Absolutely no need to spend extra money to develop a MEAN app on those OS, I recommend to stick your your computer for development.