Is mobile data (eSIM) more reliable than the Wi-Fi at my resort (e.g., in Le Morne or Grand Baie)?

Yes, 100%. Wi-Fi in most luxury resorts in Mauritius is famously slow, congested, and only works in the lobby. A personal eSIM gives you a private, fast 4G/LTE connection that works everywhere, in your room, by the pool, and, most importantly, on the beach (like Flic-en-Flac), where hotel Wi-Fi never reaches.