It depends. The 21" iMac line was just updated this past October, so you're not at risk of buying it and then having a new release come out in a month or two. Based on past release trends, you're looking at anywhere from waiting a few months to multiple years for the next iMac update. In that regard, this is a good time to buy.
If you get into "future proofing," then this isn't a good time to buy. You don't say whether you're considering the "retina" 21" iMac or the standard, but in either case, there's no longer an option for a discrete graphics chip and the 21" iMac lineup is using the Broadwell chipset, which is one generation old (the current is Skylake). There were some concerns about the graphics chip in the retina 21" iMac, given that it's the same discrete chip as used in the non-retina versions. I haven't read about any major performance problems so it may be a non-issue, but as with all things, the next update should bring even faster speeds... and if you follow the old advice of "don't buy Apple's first version of a product," then note that the retina 21" represents the first retina version of the iMac at that price.
My personal advice? If you're even considering upgrading, this is a fine time to do it. There's always something new and great on the horizon, but we're also always upgrading machines faster than we realize.