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.
Think it's mainly about keeping it liveable now. Searise is a lost case anyhow for a place like Venice, 2100 or 2200 what's the difference, It'll be lost beneath the waves or hidden behind such a tall permanent seawall that the bay basically dies (and starts smelling, sanding etc). No-one is discussing really long term, at all concerning cc sealevels. Most coastal areas are just an illusion to keep dry long term (100+ years from now), there's no turning back damages done. Planning with optimistic 2100 sealevels is really short term compared to the scale of the issue.
The push back from the residents is a bit of knee jerk reaction, it’s them saying “treat our city like an amusement park, fine, pay like you’re visiting an amusement park”.
Having visited in 2019, it was incredibly clean. They charged just a standard transient occupancy tax of I wanna say a few euros a day. But that’s multiplied by 24 million annual visitors staying for multiple nights each. I need to read this article when I get home and see what the new funds are earmarked for, because I remember being happy with everything from cleanliness to safety to transit while I was there.