So are Mind and Heart the weakest aspects?

There’s no such thing as the ‘Weakest Class’ or the ‘Weakest Aspect’. You could argue, as the Master Classes, Lord and Muse are the ‘Strongest Classes’, but even then how effective a Classpect combination may be depends entirely on who is the Hero, who/what they are up against, and the specific details of their situation.

Prince may be a Destroyer Class and seemingly more powerful and potent than a Seer, but with the Foresight of a Seer, the Prince may have no chance at beating them.

Similarly, Mind and Heart are more specific than, say, Time or Space, but that doesn’t mean a Hero of Mind or Heart is necessarily weaker- I mean have you SEEN Terezi? She’s possibly one of the most Badass characters in the entirety of Homestuck, and she’s a Seer of Mind.

So no, Mind and Heart are two Aspects. Some may seem more relevant at times than others, but they all form Paradox Space together, and their importance and power depends on the scenario in question.