If there are 5 starters per team (for argumentative purproses, injuries happen and all that) for 30 teams, then ideally out of 150 guys the top 30 are your #1's your 31-60 are your #2's and 61-90 are your #3's so on and so forth.
This might be a semantics battle, but I say an ACE is your top pitcher, so hypothetically your top 30 pitchers in baseball are an ACE. Now, maybe there's a difference between the top 5 and top 25-30 that's fine. But I believe that's a fair argument because what's average is completely reliant upon what everyone else is doing. If runs per game go up and the average ERA is 6.00 then all of a sudden a guy with a 4.00 ERA is an ACE. It's all relative to what everyone else is doing.
Of course, AS fangraphs has said, league ERA changes from year to year, and I'm just looking at ERA leaders from this year. Maybe next year there are 30 guys in that group. IDK.