I'd argue that Lisa is an atheist.
Atheism is, in the broadest sense, an absence of belief in the existence of deities. Less broadly, atheism is a rejection of the belief that any deities exist.
As Seth R mentions in his answer, Buddhism is based on the teachings of a human, rather than a deity or god. Buddha isn't a god, nor is Siddhartha Gautama. The fact is, Siddhartha isn't the only Buddha, as it's simply a description of the level of consciousness a person has achieved.
A Buddha is one who has attained Bodhi; and by Bodhi is meant wisdom, an ideal state of intellectual and ethical perfection which can be achieved by man through purely human means. The term Buddha literally means enlightened one, a knower.
In fact, Buddhism doesn't exactly conform to everyone's idea of what a religion is.
Because Buddhism does not include the idea of worshipping a creator god, some people do not see it as a religion in the normal, Western sense.
The same problem exists for atheism, since it specifically doesn't believe in any gods. I can't find the meme anymore, but I read one a couple weeks ago that states that atheism doesn't really exist as anything other as a way to describe a disbelief in gods to people who do believe in god(s).
The definition of religion is a controversial and complicated subject in religious studies with scholars failing to agree on any one definition. Oxford Dictionaries defines religion as the belief in and worship of a superhuman controlling power, especially a personal God or gods. Others, such as Wilfred Cantwell Smith, have tried to correct a perceived Judeo-Christian and Western bias in the definition and study of religion. Thinkers such as Daniel Dubuisson have doubted that the term religion has any meaning outside of western cultures, while others, such as Ernst Feil even doubt that it has any specific, universal meaning even there.
Heck, I've heard people say they don't believe in the religion of science, but I think I'm getting off topic here.
The fact that Lisa is "allowed" to give lip-service to other religions simply shows that people can disbelieve in a god(s) or faith while still wanting to spend time with their friends and family. Simply not arguing with people about their faith, or lack thereof, is simply a human trait to avoid confrontation when it's either not necessary or useless.
IMO, there's very little difference in many religions, including the god(s) they worship, just in the ways they worship the god(s). So really, there's no reason a person can't be multiple religions, except for the people who refuse to believe the god(s) are the same or that the only difference is in ceremonies. Buddhism really isn't so different from Judaism/Christianity/Catholicism/Islam. (And yes, Judaism, Christianity, Catholicism and Islam all worship the same god.)
Also, most holidays (even the religious ones) are more about coming together as a family and friends than anything else, anymore. Passing around gifts, good food, conversation, and generally good times is more about community than a specific deity. This fits well with Buddhism, Atheism, and a whole host of other religions.
What Lisa really needs is the same thing a lot of people need (and likely why there's an episode about it): that's a way of understanding that their beliefs or lack of belief in something is just as valid as anyone else, and that a disbelief in gods isn't demeaning nor does it mean that you have to argue with everyone about religion all the time. There really is a way to co-exist.
The only problem for that comes from religions that refuse to believe in facts or the rights of others, but that's far beyond the scope of this Question. I'll only say that Lisa being Buddhist is slightly more acceptable to Christians than her being an Atheist is because of their misconceptions that Buddha is a god and somehow believing in a god makes it more likely they can be converted back.