just judging by the PP standings I must assume she has a lot less supporters
Somewhat, yes - though not by anywhere near the margin those standings would suggest because a lot of it is inherited from the Powerplay 1 position when Kaine didn't exist as a Power at all and when Power size was much less correlated with supporter counts. Since the start of Powerplay 2, Kaine has actually gained slightly more new systems than Mahon has (though Mahon has successfully significantly reinforced more of their existing systems and seems more active overall)
But in all cases 90% of the participants in an unrestricted CG are either going to be from other Powers or none at all. Winters (11th) beat Grom (4th) without any difficulty a few weeks ago, despite Grom being both slightly larger and making more net progress in Powerplay 2. It's really "everyone vs everyone (which has always had a significant Mahon bias)" with specific Kaine and Mahon pledges playing a fairly marginal role [1] either way.
Ultimately the problem is that "competitive" doesn't really work at all with the CG format. Even if there's no quantitative reason in terms of either CG difficulty or rewards to pick one side or the other, people will just "like Mahon more than Kaine" or whatever and if there's a 3:1 bias of that in early players that's going to be representative of the final result so there's not going to be any suspense.
I think there have been just three pairs of competitive CGs where the result has been reasonably close, even then only one of those had actual lead changes after the first few hours, and none of them were at all
quantitatively equal in terms of how easy it was for each side to proceed - indeed, the one with the lead changes only happened that way because of something which would widely be regarded as a design flaw...
[1] From the "you were Kaine's 324th-best agent, well done" weekly messages I get, which give a reasonable estimate of how many active players a Power has in any particular week, I think probably >80% of the Kaine side probably are neutral or others, and >90% of the Mahon side likewise.