If that’s all the money the city needs to clean up after tourists I think it’s extremely reasonable. There are a lot of cities that already impose hotel taxes that are significantly higher than this amount.
There’s been a big pushback by residents about the commoditization of their city but, to be honest, Venice itself is a tourist attraction and can benefit the regional economy a lot better if that reality is accepted. If the city would like to declare an isolationist policy and bar tourists completely it’s certainly an option - but the infrastructure required to preserve it through climate change is far beyond the means of the local economy.
It’s not particularly fair but if we want the city to continue existing it needs to pay for a lot of infrastructure to combat rising sea-level and, especially, increased variability due to storms.