No and yes.
You asked two entirely different questions.
Do mobile homes generally appreciate in value? No. The structure generally loses value. However, the next question is whether the home is in a park in which you're just leasing the land (a land-lease or ground rent) or whether it's somewhere where you actually own the land. If you're just leasing the land, then you have a depreciating asset. Period. If you own the underlying land, the land might go up in value, perhaps enough to offset or more than offset the declining value of the structure.
Are they a good investment? They can be fabulous investments. Absolutely. You can buy them for all cash, mark the price up a bit, and sell on terms. Google "Lonnie Scruggs" or "Lonnie Deal" for more information. Or, if you have the ability to rent them out, they'll often cash-flow very nicely. Or sometimes they do go up in value. Generally their value declines, just as a car's value declines. But their value is also determined by the cost of other rentals and sales of real estate in the area. Example (from real life): Suppose a 2 bed/1 bath apartment rents for $1,200. Suppose a 2 bed 1 bath condo in the same area sells for $170,000. In that case, you might find a mobile or manufactured home for $20,000. Now: What happens if real estate prices rise? (Maybe general market conditions improve. Or maybe a new company moves into town and housing becomes tight.) The apartment might now rent for $1,500. The condo might sell for $210,000. Well, what do you think happens to the value of the mobile/manufactured home? It'll go up in value, too. Maybe to $30,000. It doesn't sound like too much, but that's a 50% jump in value.
Plus, if the mobile home is saving you money every month over renting a stick-built home, or buying a home, then that's return on investment, too. And suppose you're saving $300 a month. That's real money that you can save, invest, or spend.
So, mobile homes aren't right for everyone. And, no, they generally don't appreciate in value. But they can be very, very good investments and in the right situation they can absolutely be the right decision financially.
Hope that helps.