Using runes mentally is better than nothing, but it is much better even if you just whisper them. Any kind of vibrating sound is more powerful than doing it mentally. But if you don't have an other option, doing it mentally is fine. But all your focus into the mental feeling of you strongly vibrating these words, and you can instill a very strong intention and energy. This would be the correct way to do it mentally which is much stronger than just regular thinking.