it is not necessarily bad but if its a fractional rig just let the backstay off, if its masthead tighten up the aft loweres a bit. Fractional depends on mainsail cut generally speaking masthead rigs should be kept as straight as possible (this helps keep forestay tight and reduces sag)

Lots of boats, often fractionals, have quite a bit of mast bend. Rumble will know more than me but I got told by a mast maker and a designer that they liked to see a mast bending forwards in the middle as it made it easier to hold in place.

Usually inverting a mast is a bad idea. I haven't broken many masts but most of those I have have bent backwards in the middle. It is hard to bend the middle of the mast forward too much with the main up as the sailcloth will help stabilise the middle of the mast pumping forward. The cloth cannot help push though. So having the mast want to go forward in the middle and then locking with lowers seems a sensible approach to avoiding inversion.

On my 7/8 rig the designer likes about 1/2 - 3/4 of a mast diameter of prebend forward in the middle. She is pretty rock like so it seems to work for me.

You may have to put your eye on the gooseneck and look up to see any bend. That is how I tune my mast after re-rigging etc. Close one eye and gaze up along the mast track.