well ive never made a sig but they look pretty good. The mairner one is pretty sweet. The only suggestions i can make(non sig maker) is to make lighter backgrounds so you can read the writing better. Also make the writing a lil bigger.
One thing I would suggest would be to make your players bigger. You don't really need to fit the whole player's body in the sig but moreso the top half. I know it's probably harder to do with multi-player sigs but it comes out looking better that way IMO.
With text simple is usually better. The text on your JJ Hardy looks fine. Small, simple font. Don't really worry about text for now though, that's one of the harder things to get down.
One of the simplest player effects I've messed with is simply to duplicate your player render and on the duplicate go to Filter > Pixelate > Fragment. Set that layer to color dodge or linear dodge. If it looks too bright just lower the opacity on it until it looks good.