Grafana is not just for monitoring but its focus is Time Series data. If the data you are storing in Cassandra is time series data, then yes Grafana could a good choice for visualizing that.
I’m not sure exactly what your scenario is. If the reason you are saving time series data in Cassandra is due to large volumes then you should take a look at MetricTank.
If you just want to visualize some data then maybe KairosDB will work.
There is a PR for Cassandra support that we did not merge (trying to keep core Grafana lean and small): https://github.com/grafana/grafana/pull/9774 We are going to introduce backend plugins in the near future and then this could be used (or you could run a fork of Grafana with this PR).