Could be abbreviated as a one-liner:
m_isTlpInstalled = !QStandardPaths::findExecutable(QStringLiteral("tlp")).isEmpty();
or even more concise with syntactic sugar:
m_isTlpInstalled = !QStandardPaths::findExecutable(u"tlp"_s).isEmpty();
Otherwise LGTM, but I'll let @meven do the approval thing as it was his comment.