Has the small meteorite that hit Webb done a lot of damage?
The TL;DR answer is an emphatic no. Details follow.
How seriously must we take this?
The TL;DR answer is an emphatic not at all, at least not for now. Details follow.
The four detected micrometeoroid impacts have done negligible damage. The article to which you linked answers the question. Quoting and commenting on quotes from the article,
Between May 23 and 25, NASA’s James Webb Space Telescope sustained an impact to one of its primary mirror segments. After initial assessments, the team found the telescope is still performing at a level that exceeds all mission requirements despite a marginally detectable effect in the data. Thorough analysis and measurements are ongoing.
A "marginally detectable effect in the data" does not amount to "lots of damage". The quote in the question that the impact knocked "one of its gold-plated mirrors out of alignment" does not appear in the NASA statement. That appears to be an interpretation of the NASA statement by Reuters.
That the telescope "is still performing at a level that exceeds all mission requirements" also does not amount to "lots of damage". The damage was minimal, and if all that it did was to knock a mirror segment out of alignment, that is completely recoverable.
Every spacecraft suffers glitches, most of which are minor but some of which are major. "Lots of damage" would correspond to a major glitch. This was not. This micrometeoroid impact was a minor glitch, and most likely a rather minor and fully recoverable glitch.
That "thorough analysis and measurements are ongoing" is not a cause for alarm. It's what spacecraft operators do when confronted with any glitch in the spacecraft, even rather minor ones.
Micrometeoroid strikes are an unavoidable aspect of operating any spacecraft, which routinely sustain many impacts over the course of long and productive science missions in space.
Below is an image showing micrometeoroid / orbital debris (MMOD) damage to the Hubble during the second Hubble repair mission in 1997. The micrometeoroid or orbital debris that caused that damage passed completely through the mirror. The primary mirror was never replaced during subsequent Hubble repair missions. Despite this damage, the Hubble has continued to deliver very high quality in the 25 years (and probably more) that followed this impact. One MMOD impact is not something to worry about.
Portrayed below is an image of the radiator on the Hubble telescope's Wide Field and Planetary Camera 2 (WFPC2).
The Hubble telescope's Wide Field and Planetary Camera 2 (WFPC2) was installed in 1993 during the first Hubble repair mission. This was the camera that saved the Hubble. The WFPC2 was replaced with the Wide Field and Planetary Camera 3 (WFPC3) during the fourth Hubble repair mission because camera technology had improved immensely during the intervening 15.5 years. The WFPC2 was brought back to Earth for studies. What is shown in the photo below is the WFPC2 radiator after the numerous MMOD impacts were drilled out for study. The size of the holes greatly magnifies the magnitude of the impacts. It's the number of holes that is relevant in the image.
Any spacecraft that is intended to spend years in space and that does not explicitly address the MMOD problem is a spacecraft that is not worth flying. The design review processes will ding the spacecraft designers a lot harder than a micrometeoroid will ding the spacecraft if the designers have not addressed MMOD in the spacecraft design. The review processes that follow design review will similarly ding the spacecraft build and test reviews a lot harder than a micrometeoroid will ding the spacecraft if the builders and testers have not addressed MMOD in the spacecraft design.
Impacts will continue to occur throughout the entirety of Webb’s lifetime in space; such events were anticipated when building and testing the mirror on the ground.
Fortunately, the JWST designers did address the MMOD problem, throughout the design, build, and test processes.
“With Webb’s mirrors exposed to space, we expected that occasional micrometeoroid impacts would gracefully degrade telescope performance over time,” said Lee Feinberg, Webb optical telescope element manager at NASA Goddard. “Since launch, we have had four smaller measurable micrometeoroid strikes that were consistent with expectations and this one more recently that is larger than our degradation predictions assumed. We will use this flight data to update our analysis of performance over time and also develop operational approaches to assure we maximize the imaging performance of Webb to the best extent possible for many years to come.”
This recent impact caused no change to Webb’s operations schedule, as the team continues to check out the science instruments’ observing modes and prepares for the release of Webb’s first images and the start of science operations.
The cumulative damage from MMOD will eventually and inevitably result in severe degradation of the telescope. This is unlikely to happen for a decade or two. Space is not easy. However, the vehicle was designed well and these first impacts caused very little damage.
Regarding the last quote, a commenter thought that this means the team is continuing to assess the impact of the micrometeoroid. That is not the case. Reiterating from the block quote, "This recent impact caused no change to Webb’s operations schedule." The JWST is not quite ready for prime time.
There always has been a planned long delay between arrival in the halo orbit about Sun-Earth L1 point and becoming operational. The alignment and checkout processes are very detailed and very time-consuming, about six months long. The JWST team is proceeding with the checkout procedures that need to be performed prior to attaining operational status. Those checkout procedures have nothing to do with the micrometeoroid collisions.
The image below shows very precisely what checkout procedures still needed to be done as of 8 June 2022. Each of the four instruments on the JWST has multiple modes of operation. Each of those operational modes must be checked out and signed off before the instrument is deemed to be operational. As of 8 June 2022, only 2 of the 17 instrument modes had been fully confirmed and signed off. The final commissioning checkout procedures of those remaining 15 modes is what the article was referring to. The goal is to have all of this commissioning work (and signed off) completed by 12 July 2022. This checkout has absolutely nothing to do with the micrometeoroid impacts.