Is “the best thing since sliced bread” supposed to be taken sarcastically?

It is a humorous or ironic exaggeration rather than sarcastic: it implies that what you created was excellent of its kind, even if it was not an earth-shattering invention.

The phrase (more often said in the US as “the greatest thing since sliced bread”; but ‘best’ has overtaken ‘greatest’ in BrE usage— see Google Ngrams) has its origin in the advertising slogan of the first company to market pre-sliced bread:

Otto Frederick Rohwedder of Davenport, Iowa, USA invented the first loaf-at-a-time bread-slicing machine. A prototype he built in 1912 was destroyed in a fire and it was not until 1928 that Rohwedder had a fully working machine ready. The first commercial use of the machine was by the Chillicothe Baking Company of Chillicothe, Missouri, which produced their first slices on July 7, 1928. Their product, “Kleen Maid Sliced Bread”, proved a success. [...] The bread was advertised as “the greatest forward step in the baking industry since bread was wrapped.” —Wikipedia, s.v. ‘Sliced Bread’; my emphasis

ADDED:
Drew points out, quite correctly, that I have too easily accepted Wikipedia’s assertion that the Chillicothe Baking Co. tag “led to the popular phrase, ‘the greatest thing since sliced bread’”. The actual origin is both more complicated and more obscure.

What actually happened was that pre-sliced bread, backed by aggressive marketing (see the ads below, from 1929 to the early 1930s), was an immediate and overwhelming success with consumers. Various sources report that by 1935 between two-thirds and 80% of bread sold was pre-sliced. Sliced bread was clearly a great thing.

The earliest version of the “since sliced bread” which anyone has located is the bottom ad below, from the Feb 16, 1940 Burlington Daily Times-News, in which Southern Bread’s announcement of a ‘twin-pack’ (a loaf packaged as two half-loaves) is accompanied by ‘testimonials’ including one from ‘Mother’: Greatest convenience since sliced bread.

An episode three years later suggests that Southern Bread did not invent this out of thin air. In 1943, during WWII, the Federal government imposed a ban on sliced bread, to conserve the steel going into slicing machines and the heavier waxed paper required by sliced bread. The public was outraged. Wikipedia quotes a letter to the New York Times:

I should like to let you know how important sliced bread is to the morale and saneness of a household. My husband and four children are all in a rush during and after breakfast. Without ready-sliced bread I must do the slicing for toast—two pieces for each one—that’s ten. For their lunches I must cut by hand at least twenty slices, for two sandwiches apiece. Afterward I make my own toast. Twenty-two slices of bread to be cut in a hurry!

The ban was lifted in two months.

The “greatest thing” piece of the quote emerged within a very few years. Partridge, Dictionary of Slang and Unconventional English, says “Services’, mostly army, ironic c.p. for any useful novelty: since mid-1950s. By 1980, > more widespread”. The earliest use I have found is a Google Books snippet from Nathaniel Benchley’s Side Street, published in 1950 “That Hermione,” Hale said, when the outer door had closed. “She's the greatest God-damned thing since sliced bread.” Benchley had served in the Navy in WWII.

But for what it’s worth, in another story in the same book Benchley also writes “She looked as though she'd be the greatest God-damned thing since 7-Up”. 7 Up was another marketing success story: the drink was launched by the Howdy Corporation in 1929 as “Bib-Label Lithiated Lemon-Lime Soda,” with a winged logo echoing the description “Seven natural flavors blended into a savory, flavory drink with a real wallop.” The cumbersome name was subsequently shortened to “7 Up Lithiated Lemon Soda” and eventually to just “7 Up”; it was so successful that by 1936 the company itself changed its name to “The Seven-Up Company”. In the late 40s 7 Up was the third-best-selling soft drink in the world.

Perhaps Benchley, who was born in 1915 and witnessed the extraordinarily swift rise of sliced bread and 7 Up, should have the credit for inventing this catchphrase.

^{Image Sources:
Chillicothe Baking: futurechallenges-dot-org (A link to this source is "not allowed" by SE.)
Mrs. Baird’s: [http://lospastelesderosa.blogspot.com/2012/11/pan-de-molde-con-cara-de-buho-origen.html]
Butter-Krust: [http://mysapl.wordpress.com/2014/10/08/october-8-in-san-antonio-history-5/]
A&P Grandmother's Bread: [http://www.pinterest.com/genealogybank/old-newspaper-ads/]
Wonder Bread: [http://lincolnliontales.com/721/oped/a-dive-into-the-past-1920s/#prettyPhoto] and [http://www.pinterest.com/thewonderbread/our-wonder-ful-history/]
Burlington: [http://www.firstmention.com/slicedbread/]
7 Up: [http://jaced.com/2005/06/23/the-history-of-7up/]}