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Let $C$ be a locally presentable category. Is it true that the category of pointed objects $C^{*/}$ in $C$ is also locally presentable?

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Yes. The category of pointed objects is the category of algebras of an accessible monad on $C$, namely the monad $c \mapsto \ast \sqcup c$ (that is induced by the unique monoid structure on $\ast$ with respect to the monoidal product $\sqcup$), and such a category of algebras is again locally presentable. See Adámek-Rosický, Locally Presentable and Accessible Categories, 2.78.

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Over $C/K$ and under $K/C$ categories of a locally presentable category $C$ are locally presentable for every object $K$. See corollary 2.44 (and 2.47) in Adámek-Rosický, Locally Presentable and Accessible Categories.

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