You have to play each champ differently for every lane and that applies in the jungle.
Leona has low damage, sucks at clearing monsters, will be quite slow at clearing own jungle but she is tanky as fuck, as a bot lane support if possible golems should be taken with shield because Leona will take minimal damage (unless going for level 2 E/Q kill combo).
Your initial jungling clearing will be slow but there are other champions which are decent junglers which have the same problem, Poppy comes to mind. Leona jungle won't be levelled quicker as much as regular junglers due to slow jungle but she will be more levelled then if she was just a bot support which brings Leona's great strength, she is a brilliant tanky initiator being able to get to enemies through minions, then stun them and all the while being so damn tanky, no one else really has that much potential without using ults and if Leona uses ult she gets even more stun damage.
Now all this is reasons why Leona is a super support but all the tanky CC supports out there (Blitz, Naut, Mao, Leona, Ali) can make powerful gankers as well. Ali is a nice example of not really being great at clearing out camps and jungling quickly but when ganking he is amazing.
Honestly I feel the skill of the jungler is more important then just saying which champ is good at jungling, if you know how to use your champ and look for/make opportunities then lots of champs can be used for jungling.