I have questions about the online database tool Fieldbook. I've tried Webapp.SE before but the userbase over there didn't show any interest about it. Is dba.SE a proper place to ask questions about online tools?
Anything is possibly, but you probably won't find much interest here either even if your question might be on topic. We do allow questions about MS Access after all, and I don't think many DBA's are hired to manage MS Access databases. If the question relates to real database functionality/data modeling then I think you have a shot. If the question is about the product's UI I'm sure it will be considered off topic.
All of the marketing on the page you linked to screams that the product is for nontechnical people. As such they are probably going to actively prevent you from doing anything advanced. IMO those types of tools are typically marketed to people/companies that can't afford a DBA or aren't ready for the investment needed for a more robust solution.
Please don't interpret my comments as disparaging that product. I know nothing about it beyond their marketing page. Those kinds of products do fill a real need in a cost effective way. Like they say I'm sure their system is much better than managing a host of spreadsheets. However, it is trying to solve that problem for nontechnical people, not the problems that a DBA routinely encounters. Just like SQLite it is trying to solve a different use case than the SQL Servers of the world. However I think the functionality that SQLite offers is much more appropriate for DBA.SE than Fieldbook.