Is it okay or recommended for performance/load balancing to point the A Record for your mobile subdomain to a separate server, with its own IP address?
I'm assuming you're meaning you have something of the form
There is nothing inherently wrong with these being served from different IPs for load balancing or other purposes. It's not going to be an intelligent distribution of traffic based on actual load - but if it meets your needs : then as a cheap and easy solution I say go for it.
Is it recommended? I wouldn't really worry about this so much as does it meet your needs? If it does, great.
As a generic "enterprise" solution it's not a complete solution in that it has no intelligence. If 99% of the traffic is hitting one server and the other is idle that's not a very effective use of resources. Typically in a enterprise solution you'd have something out front to do the load balancing - NGINX, Netscaler Proxies, Apache, even IIS with ARR/URL Rewrite - configured in a high-availability pair or more, and then in which you define your web farms behind them and what rules are to be used for traffic distribution and affinity. This solution also allows you to scale by adding more servers to your web farms if you need to. If you don't need the extra headache and expense though this is just overkill.