Are cloud storage services a good strategy to protect against ransomware attacks?

I'm not sure about Google Drive, but Dropbox provides a way to recover previous file versions, a feature that wouldn't be impacted by the ransomware, since it relies on a file copies on the Dropbox servers. So it'd certainly be a way of protecting your data.

However, recovering everything over your internet connection is a relatively slow process. Personally, I would use a NAS device, but wouldn't map it as a network drive (because those can - and will be influenced if a ransomware is activated on your computer). I would use it via FTP / SFTP, probably with a script that syncs the files on a regular basis. This way you have the files locally, which makes restoring from an attack less of a problem. It is probably cheaper too.

Also, if you prefer Dropbox-like experience, you might want to try ownCloud on your own device - it also keeps the previous versions of file, allowing you to roll back in case of file damage or corruption. Keep in mind that storing multiple old versions of a file takes space on your NAS's disk(s).

Simple, cheap and relatively scalable solution
(Although I'm aware it has nothing with online storage to do)

I have two USB drives that I rotate regularly (you can add a reminder in your calendar if you're afraid to do so). You can use one of the many synchronization tools to choose which folders should be copied, I use Allway Sync.

One of the drive is always offline. You could even move it to another location to make your data resilient to burglars visiting your home or fire or whatever.

You can encrypt the drive too if you don't want other people to tamper with your backup. I use VeraCrypt.

Notes

The more often you rotate your drives, the least data you will lose in case of ransomeware infection. But of course that's the downside of this solution, that you need to manually rotate your drives every now and then.
But it's a cheap, flexible and effective solution against many problems that could occur.

A simple solution

To me, it matters to keep the solution simple so that it cannot be misused. For example, the NAS solution will only work if no one ever mounts the drives. I can easily see how this could fail with unexperienced user that don't know exactly what they do.
Plan for that day when you sit and try to solve a problem. One solution is to mount the drives and you totally forgot about your backup scheme that you set up several months ago.

At the time of writing, Dropbox would be a good way to mitigate ransomware attacks because a 30 day version history of file changes is kept on their servers (even on the free tier).

This, depending on the volume of data, requires a fast internet connection for both upload and download for it to be effective.

However, (big caveat) it wouldn't take much for new ransomware to be engineered that grabs your session token from Dropbox.com, or that installs a keylogger in order to capture your cloud provider password and then proceeds to select the "Permanently Delete" option, rendering the files irrecoverable.

The same goes for any online storage options, whether mapped as a drive or not, as ransomware could easily be engineered to search out SMB shares on the local network and encrypt files there too. The only real option for online backup would be if you had a write only option where the network protocol will allow new files, changes and deletions only with full version control and no possible way to disable the version control or to permanently delete past copies.

This then leaves offline backups as the final option. These would have to be manually initiated to removable media, which would be best stored encrypted off-site for protection against non-malware threats (e.g. fire or theft).