Let's start addressing the elephant in the room: can this show be good with three-and-a-half-minutes-long episodes including ED? Yes, no doubt: it's a slice of life, it hasn't a dense complex plot which has to be carefully developed in order to be enjoyable; it's just 2 to 4 girls mountaineering, everything that has to be said for plot purposes fits perfectly in the short span of time that the anime is allowed to occupy, with plenty of room left for the enjoyment of the cuteness of the characters and of the beauty of the scenery (which is good enough for a scenery-centered anime although not great). If on the other hand you are wondering how a show about mountaineering can be cute, let me reassure you: all the characters are very sweet, and honestly in a world where loli/basket anime Ro-Kyu-Bu! exists anything can be moe (by the way Yama no Susume's Kokona and Ro-Kyu-Bu!'s Hina, who have the same role in their shows as the "loliest" girls, have also the same seiyuu).
Overall if you're interested in the genre the runtime should not discourage you: Yama no Susume is a very good choice, and if at the end you're feeling like you would have wanted more of this, well, go straight to the second season (it's as good as the first, and much longer!).
