I'm afraid not. Just straight off the top of my head, Mighty Mouse dumped a whole load of cats who had been terrorising his city, By "First Cartoon Character", I meant "First Cartoon Human" (barring snowy). I am sorry to hurt and don't mind please.
Definitely it's Tintin and Captain Haddock who first set foot on the Moon and our Haddock had even dared to set f(loat)oot on space!
Moderator Note: Your confidence is admirable, but doesn't actually seem to be borne up by the facts, which weigh against you.
For example in
the 1915 Essanay animated cartoon "He Resolves Not to Smoke", part of the Dreamy Dud series, Dud is carried up to the Moon and left there by a ghost/ smoke-creature, for having been caught with a stolen pipe. However this was to an anthropomorphic crescent moon, which wasn't much bigger than Dud, so you might not count that either.
In which case
the 1917 Walter Lantz outing for his "Happy Hooligan" character, A Trip to the Moon is another early voyage off planet. The Moon is a full size satellite in this, and the Man in the Moon in this one isn't the Moon itself, but an old man who lives there.
Can't say that these are even the earliest cartoons, just ones which are certainly earlier than Tintin & Cº getting there!
The Astronomical Tintinologist Team