It's not an oxymoron, nor is it irony as far as I can tell. It would be a paradox.
Actually there are three criteria to the oxymoron. One is that it must seem contradictory, the second is that it must be true in a sense nevertheless and the third is that it's an adjective-word pairing:
Oxymoron A rhetorical figure, in which an epithet of a quite contrary signification is added to a word; as cruel kindness.
The American Dictionary of the English Language by Noah Webster (A.D.E.L.)
In case you have not realized it, two of the criteria I gave are found within the word epithet as A.D.E.L. defines it. In any case, this fails the third criterion.
Irony is when you say the opposite of what you mean, usually given away by forms of expression other than your words:
A mode of speech expressing a sense contrary to that which the speaker intends to convey; as, Nero was a very virtuous prince; Pope Hildebrand was remarkable for his meekness and humility. when irony is uttered, the dissimulation is generally apparent from the manner of speaking, as by a smile or an arch look, or perhaps by an affected gravity of countenance. irony in writing may also be detected by the manner of expression. — A.D.E.L.
There is no way for me to tell here if this statement is ironic or not, but if it is true, then it is not ironic. A wink emoticon or something of the sort would go a long way towards telling me if a statement is potentially meant to be ironic.
There's no reason it can't be considered a paradox though. It even seems like an accurate description.
A tenet or proposition contrary to received opinion, or seemingly absurd, yet true in fact. — A.D.E.L.
An opinion, in Noah Webster's primary definition of the word is essentially something that is thought to be true based upon some evidence without certainty. The popular opinion is that people laugh because they are happy, since this is the most often reported reason, but the true reason any single person responds the way they do is more of a subjective manner, than one that can be objectively evaluated with any degree of certainty. Thus the stated behavior, if true, can thus be considered paradoxical. It does not even seem to be so unreasonable that a laugh could be induced through such feelings as they are sometimes described as being induced through nervousness instead, such as in the exemplary sentence "he laughed a nervous laugh" from Collins Spanish Dictionary — Complete and Unabridged 8th Edition.
This definition of paradox seems to be corroborated by the 1913 Webster's Revised Unabridged Dictionary and the 2010 Randomhouse Kernerman-Webster's Dictionary, if you mistrust a nearly two century old lexicon. Worth note is that this use of the word seems to be losing popularity, as it is listed dead last, but it seems applicable.
Despite being an accurate definition, the word has gained many other meanings other than its most appropriate signification, so if you do use the word this way I do suppose there will be some objection based upon what somebody else told them the word is supposed to mean. Something like a sort of riddle or a actual contradiction, rather than just a supposed one. However, I do not consider this problematic when categorizing the statement in question as a paradox, because even if a word acquires new meaning, it does not necessarily invalidate older meanings. If such was not the case use of the word Heart as the seat of emotion would invalidate use referring to the organ. Even assuming such definitions are correct, their applicability would not be mutually exclusive to this one. It just would not be a paradox in that specific way.