The ABIM scores are not used, as one cannot take the ABIM exam until done with medicine residency. Cardiology fellowship interviews take place during PGY2 or PGY3 years of IM, so the applicants haven't taken the ABIM exam yet. Also, the programs don't have access to your ABIM scores because ABIM doesn't send them, and I've never seen a program ask for them, as it would be highly discouraged (or prohibited?).
Some fellowship programs specifically do not ask for USMLE scores and do not want them sent to them. At those programs your USMLE score would have no impact as they don't use them.
Some fellowship programs do use them/ask for them to be sent, and you can assume they are being looked at. I would say it is medium importance, probably high importance at some programs. If it's Harvard, Duke, Emory or UCSF then you can be most of the fellows have high Step 1/2/3 scores along with research and everything else. If it's a community cardiology program or random state U. program, the fellows probably have scores more like mere mortals (i.e. 220 or so) but it's just one factor among many used for selection. I think some programs probably have score cutoffs, but they don't tend to be as blatant about it as some specialties do when you are a MS4 med student (i.e. you won't get them to admit they throw out everyone with <230 USMLE score).
If I was someone with crappy USMLE scores but an otherwise strong application, I'd look for programs that don't request the USMLE score to be sent. Similarly, if I didn't have a lot of research or maybe didn't go to a well known residency, but had kick-ass USMLE scores, then I might zero in on some of the programs that DO require applicants to send their scores. I know Baylor used to want the scores sent and their PD mentioned during our interview day that we were picked to interview there due to our academic qualification, including USMLE scores. I don't know how heavily it was used or how it compared w/residency evaluations on his list of what was important, though....