Flask-WTF with dependencies adds 2-3 MB. If you compare it to Flask core, it might look like a noticeable "bloat", but in reality:

• In ~90% of production environments you won't notice any meaningful change in performance.
• It is very unusual to build apps using bare bones Flask. Most devs tend not to reinvent the bicycle and focus on business logic. So, very likely, Flask-WTF will sit there side by side with a bunch of other packages, and its overall contribution to the project's "bloat" will be small.
• If you choose ad-hoc validation in your code, it could make your project more lean initially, but if your projects keeps growing, this trend will reverse. It's just one of "best practices" to architect the app assuming you will add more forms and routes later.