The FPS Bypass tool is recognised by the Geometry Dash Community as a useful tool, for the reasons you yourself pointed out.
This tool has become a widely-used in the Geometry Dash community, and a lot of big names in the community are using it (e.g. Sunix and Skullo).
However, its legitimacy comes into question when it comes to the official competition - the Demon List, to the point where some users have been banned from the competition for using it.
Although, it has been very controversial as to whether this should be allowed to be used or not, as the Demon List has been banning people who have been using it. A large portion of the community does think this is allowed, though. However, the creators of the FPS Bypass have said that it should be considered a hacking tool and nothing but a hacking tool, which really started the controversy.
So, as far as using this tool goes, yes it is "legal", but only outside of the official competition.