Droplets are created to span across the whole allocated disk. But you can shrink the main partition on the droplet (vda1) and free up some disk space to further partition it. There’s an article on how to do this: https://www.ismay.ca/2015/10/repartition-a-debian-digital-ocean-droplet/
But I don’t recommend it. There are so many things that can go wrong - namely data loss. If this is a test server and you don’t care about the data, by all means, break a leg! Otherwise don’t risk it.
You can also add a volume, partiton it, and mount it. But /tmp is such a frequently used folder that performance could be affected if you use a remote block storage.
My recommendation is create a block device on top of your FS. It’s detailed in this article: https://linuxfundasite.wordpress.com/2016/08/22/increase-the-size-of-disk-space-tmp-usrtmpdsk/
You’ll probably need to stop all services that make use of /tmp so you can safely back it up and create the new partition. But this is much safer than anything else.