According to google's ngram viewer, the phrase "it's all relative" started to appear significantly in print in the late 1920s, had a little peak in the 1950s, then became much more popular from the early 1960s onwards. This only looks at its use in print - it may have been popular in spoken English before that.

However, the earliest citation I can find for "it's all relative" is from 1812, as quoted on the University of Pittsburgh's Keywords blog. Although they sadly don't give a source for this, the rest of the article is sourced mainly with the OED, which I don't have access to today to check.

So whilst the phrase is older, its popularity in print came after Einstein published the theory of relativity. There is no evidence that the theory influenced use of the phrase though.