Is It Normal Or A Convertible Frame To Flex A Bit When Jacked Up?

Well, yes and no. So lets clear up the muddy answer.
The unibody car is a wonderful design but not without if faults, as you probably know they where nicknamed flexable flyers.
First, no, it is not supposed to 'bend' much but after many miles with road conditions less than perfect they, well, loosen up in places, the problem is magnified by removing the roof, thats what the company that built the convertible did. Yes, young man, ford did not build the mustang convertible.
Heed not my friend, just grab a set of sub frame connectors, and not just any wimpy little tubes, I have seen many recommendations for maximum motorsports brand.
Now while on the subject of sfc, a good shock tower brace is recommended, more so with a vert because of the cowl/windshield area is largely unsupported, if you look under your vert you should see an extra set of braces that are bolted to the k member that are not present in coupes and hatchbacks but there is a big unsupported box at the top of the engine compartment that should be triangulated and thats where the shock tower brace comes into play. They make lower braces that are better than the wimpy stock ones too,
Proper sfc and engine compartment bracing will give you a quiet, rattle free ride for the life of your vert, not to mention add safty in a frontal crash.
And I like to talk alot, although most of what I say is nonsense and not to be taken seriously, I do have experience with a lot of verts and I love em.