Undervalued by whom?
I would say that many of the single title winner titles are undervalued by a great many watchers of the sport - whther that is fair or not is individual interpretation.
I say it however as people will see a failure to win a 2nd ast he first being a fluke as there is no consistency around the rider - does not make it right or wrong.
Absolute true followers of a sport would rarely undervalue a title win as we have watched the sport for long enough and seen what is required to be at the level, never mind to be comeptitive and then to win. But not everyone has followed the history of the sport but rather look at the printed numbers and make assumptions.
Some look and will say shortened season so does it count, where we also look and acknowledge the challenges of that season with the condensed format and its other challenges. We also, as followers of the sport recognise that the old adage is 'you can only race those that turn up' is true and no title should be besmirched because of smaller fields or a missing genius as even genius have bad days, weeks and months.
Then there are also the 'other type of watchers' that need to lessen a riders achievements when it is compared to the rider they follow or prefer. Different to the tall poppy syndrome is the sheer fact that for some, there is only one person worthy as the idolise the individual and do not acknowledge or recognise challenge.
Mir and Hayden won titles with few race wins - so what - they were consistent at a time many were not and that fact alone needs to acknowledge their title. The fact that others were inconsistent alone should illustrate the worth of the title by consistency, as the question needs to be asked why others fell or did not finish when Mir/hayden could for example - they should not be punished for anothers lack of consistency (if that rant makes sense)