As this image indicates, the fingertips don't necessarily touch the bat when a person grips it. I can see how it would be hard to get prints. Also, if there were prints, then there might be a lot of them because a lot of people (kids, Jason, people in the store where it was purchased, etc. ) might have gripped the bat previously.
I agree that someone, esp. TM might have wiped the bat and made it harder to identify prints. But as someone noted, the probable motive for TM to wipe the bat would be to wipe MM's prints from the bat.
The wiping the bat clean of prints scenario presumes that MM was the primary assailant of Jason. It's possible, but I find that theory less likely. Given the extreme nature of Jason's injuries, it seems more likely that the main assailant was TM.
He had the size, the strength and the experience of a typical American male in wielding a bat. MM has probably never picked up a bat in her life. I believe she participated in the beating (maybe she started it with the paving brick), but I think only a man like TM could have inflicted the kind of extreme injuries that ultimately killed Jason.
In sum, the issue of a lack of fingerprints on the bat doesn't seem to be that important. The defense basically concedes that TM and MM killed Jason by beating him. The important issue is whether the defense can convince a jury that they did it in self defense. I don't think so, but there is along way to go in this trial.