Looks will always matter, but to a degree.
Looks will fade or change over the lifetime you are with your partner. You can look for clues in the elders of their family to see what they MAY mature into but even that is not guaranteed.
What is more important is their personality, is their "beauty" on the inside? Remember you are planning (although plans don't always work out) to spend the rest of your life with this individual.
It is far more important to ensure that you are compatible on a mental and emotional level than on a physical one that will fade over time. Remember the saying looks are fleeting.
Choose your partner based on commonalities. If one of you is a heavy partier and the other a stay at home bookworm I guarantee this will not last a lifetime. If however you both enjoy the same activities whether that is curling up with a book, watching a movie, playing video games together or skydiving or cave spleunking, then you will find that your relationship will last longer and be that more fulfilling for the both of you.
Terms like soul-mates are often thrown around when talking about life time partners and in general that means when two souls are in harmony it will be a lasting partnership, Notice the absence of looks or physical appearance in that regard.
Remember the knockout blonde she is at 18 will bear NO resemblance to her at 78, So why worry when beauty is only skin deep, its the mind and personality that matters and will endure throughout all the trials and tribulations that come with a lifetime partner.