The truth is that Christianity (properly understood!) says that this mortal life is of vital importance - and this importance derives-from the destination of personal, resurrected eternal life.
But the idea that this temporary and sin-ridden mortal life is unimportant apparently entered Christianity very early; and I believe it came along with the idea that salvation is only available by means of each Christian adhering to stringent behavioural criteria (e.g. and especially, membership of and obedience to the requirements of The True Church).
I regard this kind of (de facto) anti-Christian Pharisee-ism as ultimately a consequence of some early decisions to make (the one true) Church an essential mediator between each Man and Salvation; without which damnation was the default.
It never was metaphysically essential for Christians to be obedient Church-obeyers; but there was a practical reason why churches became regarded as essential - which was that human individuality was much less in the early years (probably at least 3/4 of the years) of Christian history.
Since individual Christians were essentially communal and social beings; in practice they had-to be Christians via churches.
But now, humans are individuals (like it or not) and it is not just possible but unavoidable that we each take personal responsibility for our faith, and behaviours. We can no longer obey external authority (such as churches) spontaneously and unconsciously; and we should not try to do so - precisely because we now Can take personal responsibility.
(*In particular, it is overdue that Christians ceased to spend their lives circling around-and-around a perpetual and unassuageable worry about their own salvation, sometimes returning to this theme every time they pray, like a hamsters trapped a wheel - or, worse, like being chained to a never-stopping treadmill of pleading, propitiation and rumination on the same theme. Read again the Fourth Gospel - is there the slightest indication that Jesus wanted us to live this way?)
I am confident that the original teaching of Jesus Christ was that anyone who truly desires resurrected eternal life in Heaven can have it, after our physical death; through the simple means of following The Good Shepherd who will guide us to that destination; and - at that time - doing whatever is required of us to follow Him.
This means that salvation is Not A Problem for those who want it, and 'believe-on' Jesus as The Way to achieve it.
The difficult thing may be getting people to want it, or to believe the claims of Jesus as The Way - although the Truth about Jesus should become apparent after death, to anyone who really wants resurrection.
When salvation ceases to be our primary concern; then we can and should focus on living well this mortal life; which (in a nutshell) means learning from the teachings, the life-lessons, that God the Creator will provide for each of us during the time He sustains this life.
In other words; the importance of this mortal life is learning from it; and such lessons are important only because they have relevance to us (personally) for eternity.
...After all, mortal life-lessons are not very important for someone whose individuality is extinguished by death - such as atheists on the one hand, and on the other those who believe the individuality is dissolved into 'the divine' after death.
Far from Christians regarding this mortal life as un-important; Christians are just about the only people who regard this life as genuinely important: as vitally important, everlastingly important; and important to both the individual and all the other 'inhabitants' of Heaven.
(*Note added - see above)