changing posts layout in ultimate member profile plugin

Here is some basic CSS which would help make things a grid on Desktop and stacked on Mobile:

.um-profile-body.posts.posts-default {
  max-width: 100%;
  padding: 0 15px;
}

@media (min-width: 768px) {
  .um-profile-body.posts.posts-default {
    padding: 0;
  }
}

.um-ajax-items {
  display: grid;
  grid-template-columns: 100fr;
  grid-template-rows: auto;
  margin-bottom: 15px;
  grid-gap: 15px;
}

@media (min-width: 768px) {
  .um-ajax-items {
    grid-template-columns: 33.3fr 33.3fr 33.3fr;
    grid-gap: 0px;
  }
}