I think people are taking the easy way out by piling on the signature being no good. I have a whole bunch of thoughts on this:
ONE
When these were discovered, people also took the easy way out, saying that the Matty and Jackson didn't look good but that the rest of the "commons" were probably fine. I get the logic of thinking a forger would hide the big money fakes by putting them with the commons, but the flip side of that is that there is no reason to think someone collecting full team signatures would get only the commons but skip the stars. In fact, it should be the opposite, so this proves/suggests nothing.
TWO
The Giants photos, with their autographs on them, were used as a newspaper premium ahead of the 1911 World Series:
1911 Frank Smith NY Giants.jpg
So, unless Frank Smith found someone to forge Matty's signature in the early fall of 1911 while having everyone else on his team sign their names, Matty should be ok as should all of the other Giants.
THREE
I am with Snowman on not putting too much stock in the Joe Jackson Museum guy's opinion. Unless he has some training in handwriting analysis that we don't know about, or specializes in the penmanship of illiterate people, I don't know why his opinion counts for any more than anyone else who can look at the handful of examples of Jackson's signature that can be found online. He may know as much about Jackson's life as anyone alive, but that does not make him a handwriting expert. (Just like I am not a handwriting expert.)
FOUR
I also don't think that the size of the signature compared to his others (
https://www.psacard.com/autographfac...oe-jackson/21/) means much. All of the other examples are on small lines in documents - if handed a large photo with no signature guidelines on it and asked/convinced to sign it, of course he wouldn't write a tiny autograph down in the corner, he would do his best to make it fit with the size of the photo.
FIVE
Smith, the photographer, was the local Cleveland photographer. He knew the players - he also got two team's worth of these types of signed photos. The Giants make sense as a World Series assignment, the Indians make sense because they were his hometown team. The Indians had Spring Training in Alexandria LA only in 1910-1911, so maybe Smith took the Indians series in 1911, liked it, and later that season suggested that he could do the same thing for a Giants World Series preview.
SIX
This is not a lone signed Jackson photo coming out of nowhere, it was in a team album all of the same types of photos by a known local photographer who knew the players and did another similar signed team series in the same year that was published at the time the photos were taken/autographs obtained. The fact that Jackson was unskilled at signing his name combined with the extra large size of the signature to match the size of the photo, the lack of a signature line on the photo to guide him all, and the smooth/glossy surface of the photo, which is different to write on than the plain paper that all his other signatures are on (especially with a fountain pen), suggest that this signature might not end up being a perfect match for all of his other signatures. (And because of his difficulty signing, his other signatures are also far from perfect matches for each other.)
As I mentioned, l am not a handwriting expert. But I also don't think that any of the reasons being put forward to doubt this signature hold water.