It's a lose/lose for Obama.
IF it was a legal wiretap, then there was a warrant, and probable cause. That would be neutral for Obama, bad for Trump.
If the wiretap found anything, that would be bad for Trump.
Trump has pretty much declassified the investigation so anyone should be able to access the tapes and find out what was going on. That would be bad for any politician, to find out what was going on behind the public facade. But it was behind Trump's facade, not Obama's, so it could only be bad for Trump.