Sun is my favorite type of weather, so this is just my input.
Ninetales-It would get powerful quickly, but my only concern is having it against other weather teams. I doubt you would try to sweep in that situation, Ninetales is crucial to having the sun on your team. If your opponent had something of their own, and you were to lose your Ninetales, then yout team could start to fall apart. Luckily, you have Cradily to absorb Aqua jets and extremespeed, while Cresselia can take mach punches pretty well. Since it's a lead, I would consider Magic Coat or Will-O-Wisp in lieu of Nasty plot. Magic coat would work well to deflect rocks from a few popular leads. If the pokemon is not something Ninetales can counter, then they'll likely set up rocks, giving you the chance to deflect them. If they can be countered by it, they will likely try to switch, giving you a chance to use that hypnosis and put their counter to sleep, making Ninetales even safer. Will-O-wisp was a suggestion because of the sleep clause. Since they'll likely switch, you could depend on burn for the duration of the battle.
Charizard-I don't really have any issue with this set; if I were to use a Solarpower Charizard, it would probably be this. Consider a rapid spinner for support, or the suggestion I have for Cresselia. Taunt WHimsicott can block it, but he won't always be there to deal with it.
Cresselia-I do know that moonlight will restore more health while you're in the sun, and that makes it a great choice. This isn't necessary, but I would also consider a pokemon with wish on it depending on the opponent. This would allow you to send more health to your Charizard when solar power starts to take his toll. (It would also be great to restore his health if he has to come in while stealth rock is up) Your sub-punch Infernape would also appreciate this. Signal beam, or even hidden power [dark] might be of use to it over shadow ball, so you aren't' blocked out by some common threats. HP dark isn't as strong, but still deals with psychic types and covers ghosts; Signal beam would deal with Psychics and darks while also giving a chance to confuse, a nice little extra for it.
Whimsicott-I absolutely hate these things. Setbuilding for it is pretty easy, so there's not much I can say about it.
Infernape-Man, I love this thing. I would say consider a priority move like Mach punch in case of emergencies, but the other moves seem to fit well with it. The life orb also conflicts a bit with setting up a sub, even if it's supposed to be a suicide sweeper. It would work well to bring down Heatran that pop up, as well as Tyranitar -Both of whom could present a problem to a sun team. I know you would send him in and try to force a switch on the opponent, giving him time to set up. Perhaps something like an expert belt would be useful. You would lose a bit of power, but with STAB, Iron fist, and sun (with fire punch), he hits pretty hard already. Too bad he can't learn Drain punch...
Cradily-About time somebody started using this thing. I would consider filling in some ground moves on on your team, as Heatran would just laugh at this thing. He [heatran] also benefits from the sun, so if you aren't careful of him, it could hurt your team pretty badly.
These are just a few of my suggestions. Overall, it's a very nice team. Way to not just slap solarbeam on 6 fire types instead of hidden power and call it a sun team. :)